I don't think you will be happy trying to pull an 8,500 lb trailer very far from home with a stock 3600.

If the trailer was only 2,000 or perhaps as much as 3,000 long trips would be not much of a problem.

For what you want to do I say play with the truck as is, and get a newer truck for the trailer.

To make your old truck reliably and easily pull a heavy trailer on long trips will probably require lots of modifications. It all depends on your expectations though, if you don't mind going over hills in 2nd gear at 20 mph and occasionally getting up to 45-50, if you are lucky, then you may be OK with the stock truck.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-