Mike welcome to the Stovebolt!
I'm currently building a 1955 1st GMC 1-ton to pull a trailer. So aside from a slightly different cab and 12 inch longer wheelbase, my project is very similar to yours.
You don't say what the trailer you are pulling is, or where you expect to pull it. For a number of years I used a 454 powered 1-ton 4 wheel drive to pull my 7,000 pound trailer. I crossed Wyoming able to pass at will and went up Marathon Pass with ease with that setup.
To me that sets the standard for my project. Actually I want to be able to pull 10,000 pounds at 75+ across Wymoing (lots of grades on the freeway) and then run up something like Marathon without having to slow down. My now gone 454 powered pickup set the standard.
I have a Cadillac 500 with TH400 and have installed a GM 14-bolt rear with 3.42 gears. Next step is the IFS from a '72 Chevy C20.
The GM 14-bolt is what GM uses under many 3/4- and 1-ton trucks. I also have a Dana 70 HD which is GM's other choice. I went with the 14-bolt because I like the gears it has and because it fit nicely. My Dana has 4.10 gears which would be better for starting a load, but spin the motor faster all the time and I felt the Caddy motor has the torque to move the load but would be happier turning a little slower.
The torque band of the Caddy motor with those 3.42 gears mean I'll cruise comfortably at 60 to 80, and should have enough to start any load I have any business towing.
The '72 IFS gives me disk brakes and power steering without some of the problems that come with converting the straight axle. It also allows me to remove the bellhousing crossmember without worry over structural problems.
The combination of the '72 C20 IFS with the newer 14-bolt gives me modern brakes enough to stop the truck with load.
I'm happy to find someone doing a similar project and would be happy to dialog about the choices I've made and what the pros and cons of each might be.
