5.3 suburban with 3.73~ 5000lbs(boat) and I have to use 2nd gear on most grades, slow going but 11mpg for most trips including our Ca to Canada fishing trip.

6.0 2500hd with 4.10~heavy trailer(8000lbs) pulled good till I got half way up reno. Second gear again.

tbi-350 I've been down to first gear
Vortec 350- down to first gear and boiled tranny(great trip)

I would look for a low gear ratio and big motor option, I personaly like the titan(I want to tow with one but haven't)
To date the best tow vehical I have used is our old 1990-f-250 with a 460 and 4.10 it makes lots of bad noises but still runs(I have tried killing it)

 

I would consider a Duramax/Allison Combo. Our New company truck is a 06 Crew cab short bed 4x4 with the LBZ/6speed Allison. On short trips and stuff doin the speed limit i usually see 18.5-19.5 MPG out of it with only 28k miles on it. Pulling our 16' Vanbody tandem axle trailers that weigh from 5800-6500lbs we usually get about 13-15mpg. Not to mention it rides very smooth towing that much compared to our other trucks (Dodge & Ford 3/4 tons).
And its pretty fast too...
I out ran our 02 Excursion 7.3 with Intake, Exhaust, Some type of tuner, not sure which one. I put a easy 3 lengths on it, and he got the jump... yea we play in our company trucks... My boss likes the simple Go Fast goodies.

 

I recently picked up a 04 silverado 1500 for a daily driver, future tow vehicle. Its a 4.8 4x2, but dont let the previous posters scare you away from getting one to tow, I had no problem at all towing a friends 3500lbs TA on a trailer. I'll be lucky to tow five times a year so if its not the fastest thing in the world i dont care, i'll take the milage of the 4.8 anyday, I get 17 to 20 city/highway mix.

 

Thanks all but i am looking for something a little older then an 03.

 

what cab configuration? how many miles are you looking to put on it? price range?

If I had to buy a truck to last the test of time it would be a tbi a 95 is the best year since you get the newer interior. We had a small fleet and our tbi's out lasted the company.