Several things,
You probably have 7.50x17" tires now.
I would suggest 8R19.5 tires on used 19.5 wheels from an older bread truck, or new wheels from Rickson Truck if you can't easily find used ones.
The 8R19.5 tires are about 32" outside, ans that's as big as you can fit easily on your truck, bigger tires, the faster you go.
I agree, you should have 5.14 gears in the rear now. You will need something faster to get to 70 mph. Try this site, it is helpful for figuring speeds.
http://www.drivetraindirect.com/t_gear_calculator.htm Figure for a 235 or 261 your at your cruising speed, say 70 or whatever, your engine should be about 2,400 rpm, any faster and it won't last very long, slower and you don't have power for the hills. Whatever engine you do choose the cruising rpm should be about 100 or 200 over the rpm where the engine makes its peak torque.
From my figuring, you need 4.11 gears and a o.73 overdrive, or no overdrive and 3.54 gears.
For gears faster than 4.11, or 3.90 if you are really lucky, you need a entirely different rear.
A stock 235 from about 55 or later will just be enough power to go 70, but not haul much at that speed. The easiest swap for more power is a 261.
Any more power and it's not a real easy swap.
OK, suppose you figure out how to go fast, now you need to stop. Without a load and with a dual circuit master cylinder conversion your brakes should be fine with 70 mph, any more and you should consider better brakes.
You have a lot to figure out, and lots of work if you want to take your 1 ton up to 70 mph.
I have done some upgrades to my 52 1 ton, check out the pictures on the link below.
Grigg
