the thing is that if the plenum gasket is bad it WILL give you a vacuum leak!!! it will suck air from within the crank case.

I would do the intake as a matter of maintenance just due to the age of the truck... gaskets are around $26, valve cover gaskets are around $20 and a gallon of anti freeze (if you buy concentrate and add your own water) plus a bypass hose ($6 that's the 1" hose at front of intake that leads to top of water pump, almost a must unless it has been replaced recently, if not, it is a water leak waiting to happen. is your engine leaking oil? ANY oil at back of intake? That means intake has to come off anyway for new gaskets. and at that point it's a few more minutes to flip the intake over to change the plenum gasket. the oil leaking out the front or back of the intake is no indicator of plenum condition, sure... take the lid off the air cleaner, and WITH ENGINE OFF, open the throttle fully and look inside with a flashlight. If it is coated with oil within the plenum gasket NEEDS TO BE CHANGED! do not be afraid of this job it isn't as bad as it may appear to be, I just did my 92 this past weekend.... I had to replace the water pump, and "as long as" I had the AC comp and alternator off along with their mount bracket and had the cooling syst. drained (which also coincidentally have to also come off to pull the intake) I might as well.
It appeared to have never been apart since being assembled at the factory but saying that, I found that the rear most 3 intake bolts especially on the driver side were barely snug, definitely nowhere near the 12 ft/lb they are supposed to be.
I had been putting up with that oil leak since I bought the truck 15 months and 30K miles ago....

 

if youre going to tell him how to fix something tell him the correct way.

doing just the gaskets doesnt fix the problem. Its the pan that's the problem along with the bolts being a hair too short.

Dont half *** anything. Remember what happens when someone does? (Brake booster deals eaten by a bad master just off the top of my head...)

 

Nah, that wasn't half ***....that was straight off the new MC and Booster installed together and the MC leaked within a weak. Cardone junk.

 

Ive done several plenum gaskets while putting the original bolts in and original pan and have had good success with it.
yeah I know a thin steel pan bolted to a cast aluminum intake isn't ideal... but not much in this world is perfect.... including anyone here on this forum.... not singling out anyone.

 

The plenum problem isn't due to different metals. The difference in expansion between the two is measured in thousandths of an inch. (single digit at that, yes, I did the math.)

The REAL problem was a cheap gasket, and bolts that were a touch too long. A good gasket, and shorter bolts will solve the issue just as well as the aluminum plate.

 

Just to give a warning about the oil in the intake test. I did the Hughes plenum. I found oil in the intake after. It had me perplexed. I put a filter in the PCV and the crank case vent line that goes to the air cleaner. The PCV line had no oil the crank case vent line did. I worked out that the air filters I was using were not sized correctly and the oil was blowing or getting sucked past the top and bottom of the air filter from the crank case vent. Putting those filters in caused back pressure which made some seals start leaking I had to put a pan under the truck. The leaks stopped after I removed them.

 

Just so yall all know, changing the plenum gasket, intake and rocker covers, for me ain't no biggie, the hard part is finding out the cao=use of eth work that needs to be done before you do it not doing it, I've worked on everything from lawn mower to F18 fighters and just about everything in between since I was 5, just not dodge, the deepest I ever went into a Chrysler is swapping the nylon timing gears in a 78 Fury over to steel, due to the fact I was Chevy man till a steering issue in a 05 Malibu almost cost my entire family our lives when the electric units up and quit at 55, today I'm gonna block off the vac booster and see whats what, but here's an update, I removed the electrical sensor on the back of the Holley yesterday and cleaned it, lubed it inspected the spring and reinstalled it, for a few seconds I had 18 inches of vacuum no eyeball burning smell out of the tail end , so I'm still leaning towards the outside of the engine, and being that this monster has a computer I am assuming once I get the engine right my od issue might vanish too. either way I want to do the vac issue first because I don't know transmissions that good ,like I said I was a chevy man, ya just yank those toss em and go to the junk yard for a new one.

 

that's what have been trying to say forever... this aint no terrible huge job and cost of parts is minimal... AND unless wrecked and junked very early in its life, if you work on one of these Magnums, it WILL eventually need the intake and plenum gaskets done.
you mention 78 Fury... I have one and have had it for 10 years... 2nd owner, 51k original miles. mine's a 2 door.
I too did the timing chain, original one would have been"fine" for years to come, but when the water pump went, it was only 5 more fasteners to get to the T chain, was a "as long as I had it this far apart, I might as well" kind of thing.... but yeah no more nylon cam gear to worry about. next is to get rid of the Lean Burn (easy)

05 Malibu... I ended up with a "classic" version of that car, still hydraulic power steering, had the 4-banger in it. got it from a neighbor's mother in law after the engine cradle rotted out and the steering rack busted away from the cradle so she went and bought another car. I replaced the rack and cradle then had to wait 3 months for her to apply for a lost title, Illinois sucks) just sold it a week ago finally... too small for me, too many doors, no bed for hauling and front wheel drive. and a Chevy besides... I never did transfer the plates and insurance to my own name.

 

I found the source of the 15 in of vac!!!!!
Its the little electronic thing on the back of the throttle body if I slightly plug off the hole the vac goes up immediately to 18 if I close it off too much it stalls, and if I open it more vac drops, when I achieve 18in of vac she runs so smooth its unreal so I asked my neighbor he said that art and the O2 sensor determine how much air goes in to add or subtract O2 emissions, If anybody knows what this thing is please tell me cause I have no idea what the hell its called

 

Can you post a picture of it? on my 3.9 the IAC is on the back of teh TB but there is no vacuum hole I recall.