I'm my opinion I think you're going to be disappointed. What is your mileage while towing with the diesel? your going to spend more in gas and the power is nowhere near the same. I'm not knocking the hemi but there is going to be a huge difference.

 

I've had a diesel which I used when I was tourney bass fishing most weekends from the early to late '90s. Cut down a little on the distances I was traveling and sold the truck, choosing to trailer with my second Gen 1500 RCSB 4x4. Even with the relatively large 360 engine and the fact that I was pulling less than 3000#, the gas mileage was horrible.

IMO it'd be fine UNLESS you owned a diesel and know what a diesel pulls like.

What do you consider "high mileage" on your CTD? My rule of thumb is a properly maintained CTD with 350,000 miles on it is about like a gas V8 with 100,000 miles on it.

Got a buddy in Georgia who has his own business hauling those HUGE rooftop commercial air conditioning units across the country for delivery, pulling a huge goose-neck with his '94 CTD dually. Last I saw the guy about a year or so ago he was soon to be a member of the million mile club, so I dunno if it's a good move for you to ditch the CTD in favor of a 4-5 year old Hemi...

 

I have towed a number of trailers with varying weights with my truck as well as with my old '00 5.3L Silverado. There's no comparison as far as power goes. Ram has much more. I know exactly what you mean with the GM searching for gears on mild inclines.
I towed my Mustang a few times and the total weight was just under 6,000lbs. The only real struggle that the Ram has is off the line but once you get moving, it's like there's less and less weight back there. I had no problems downshifting and picking up speed but I could def feel the truck pulling it.
I have pulled lots of wood with my dump and landscape trailers and those weights range from 5,000lbs-8,000lbs.
At 8,000lbs the Ram will struggle compared to a diesel, but if your towing just classic cars, your total weight shouldn't be more than 7,000lbs in the heaviest situations and you'll be fine especially with short distances and if your only loaded one way.

2 things.
Stock transmission programming is soft and mushy. I'd highly reccomend a Superchips Handheld programmer which has a tow tune and very advanced shift schedules that you can take advantage of. I set up my shifting to the way I like it so the truck won't search for gears or take a really long time to downshift. Keeps the power there and actually gets you better gas mileage because of it. Regardless of tuner or not...I would not reccomend towing unless pressing the tow/haul button to keep it out of 5th.
Another thing...1500's have a soft suspension. Their total payload is ~1300lbs, so I would also highly reccomend airbags for the back. I paid $220 for mine delivered to my door and they took less than an hour to install. They ride real nice if you don't put too much air in them. I use around 25#'s of air towing a car trailer loaded. truck handles much better with them. Waaaaaay better ride than add-a-leafs.
I have 3:92's with 20" rims(33"tires)

 

I have had 4 dodge diesels but my other 3 were 12 valves which I liked alot more because of how simple they were and they got incredible fuel mileage. My current one has 390,000kms but it is getting costly to maintain. I replaced a hub, transfer case and the turbo recently and what worries me is the injectors on the common rail trucks (very expensive & common failure). I wish they still made them like they used to...

 

I just returned from a 1500 km (round trip) haul of my 25 ft trailer (55-5600 lbs loaded) in my 06 quad cab shortbox - dead stock w/ 20" wheels and the 3.92 rear end. Under reg. conditions, mixed hwy and city driving, I get 17 mpg. but towing dropped me to 9. I left the tranny in D and just let it do what it wanted, O/D, direct, 3rd whatever. Daytime temps were in the mid 20's (celcius - approx. 72F) and my temp gauge never moved. Steepest hill I ran up was 6% over about 3km. Averaged 100-110km/hr but occasionally pulled out to pass a grey hair and got her up to 130 or so. Just moved from a 2001 Yukon XL w/ 5.3 and WOW, the hemi would've pulled the bumper off that unit! Is it the most fuel efficient unit on the road? No, but who cares, it's the nicest truck I've ever driven - even makes ME look better (IMHO)!!

 

Keep your CTD, AS much as I prasie the hemi, I take it all back when thiers a automatic and c clip rear ends in a 1500 Ram. Having to little truck will drive you bonkers vers having too much truck and never using it.

Even in the new 2010 2500 Rams with the hemi, the automtic is a three leg dog walking backwards, to add air bags extra leafs and tunners to a 1500 Ram still gives you a week rear diff.

 

Nothing a $550 Detroit Truetrac or a $400 Auburn posi unit won't take care of. I'd rather spend an extra $550-700 to install a nice posi unit than to spend $5-10k on a truck that wouldn't get much real use.

I'll def agree that there are times a bigger truck is helpful, but I can promise you that a 3/4ton Diesel will cost more for repairs than a 1/2ton gasser would.