I think with a five or a six speed 20mpg out of a fuel injected big block is totally doable, my 85 is a 4x4 with a carb'd 360 and a four speed manual and with my 1000pd camper in the back gets around 16mpg on the highway if the winds not howlin.

Also i dunno if they r still around but i heard a few years ago that bary grant (demon carb's) was designing a bolt on fuel injection setup

 

I would be very surprised if you see 20MPG on average from a 440 or a Hemi in this heavy a truck. I base this on my experience with the type of people that put a big block in their vehicles. Most people that would do this conversion are also the type to put their foot in it instead of driving like grandma, but only you know what type driver you are.

As far as auto verses manual I believe that decision is totally dependent on what the purpose of the truck is. I was very surprised to find out that my truck came to the dealership as a manual and was converted at the dealership to an automatic because they couldn't sell it as a manual.

 

I'm currently in the middle of a 5.7 swap for my 1990 ram regular cab short box, the stock motor with a updated ignition will make 400hp and 420 ft.lb, combined with the stock 904 and low gears in the rear end I'll get 20 mpg easily. Probably 25 mpg highway

 

I would be surprised if you get 20mpg with a automatic. What rear gears are you using? How much did you pick the 5.7 up for?

 

i get 16-17 mpg with my fuelie 318 right now, the hemi with a new ignition will get 20 mpg, rear gears are stock 3.55 or something like that.... but ill swap rear ends to a ford 9" in a couple of years....

 

I'd say - if you had a 2001-up Truck, put a 04'- 05' Hemi in it.. But with all the changes you'll have to make , a fuel injected 360 with auto trans. would be you're best bet...Just my opinion...

 

545rfe would actually be a poor option for the 5.7 hemi, there mass makes it tricky to get them under the floor, and you would have to stick with a stock ignition and computer which aren't cracked in depth yet and aren't as clean as other ignitions/computers