JD
If you want to modify a 58, get a chevy. If you want to drive a 58, get a Jimmy! The 58-59 gmc trucks will work all you want the way they are. It is not a bad idea to upgrade the brakes. In the long run the truck will be worth more the closer it is to it's original factory configuration.

If you are looking for a work truck/ hauling truck, consider a longbed. 1/2 tons have better gears for speed, 3/4 and 1 tons have better gears for work.

Step 1
Buy truck

Step 2
Drive the truck. You might just like it the way the factory built it.

Step 3
Assess what you have, and how you might want to modify it. Ask questions and look for parts. Don't start buying parts untill you understand your entire build.

Step 4
Upgrade brakes...Dual master cylinder and disc brakes are a necessity for driving on the interstates.

Step 5
To be determined:

I like the GMC trucks myself. I have had three 59 gmc trucks.
My current 59 GMC is a 1/2 ton longbed. It was originally equipped with a 336 V8 (basically a debored Pontiac 389) and a HydraMatic transmisson. The original engine and transmission were long gone when I bought the truck. A previous owner somewhere put a 1958 GMC motor and 4 speed in the truck. That engine and transmission combo was tired, so I bought a 1959 Pontiac 389 off craigslist and had that engine built to drop in. I actually chose this motor because it was the only motor I could use to give me an upgrade without having to modify anything.

So my truck now has a 59 Pontiac 389 bored .030 over and stroked to come up with 447 inches. A 1957 Pontiac tripower setup provides adequate mixxing of the air and fuel. My truck has the very rare Factory power steering setup, so the original front axle was converted to 12 inch disc brakes. A 5 speed OD transmission lets me comfortably keep up with traffic. An aftermarket 3.70 ratio rear axle allows my truck to accelerate hard or work a load.

Please note there subtle differences between GMC and Chevy trucks. Engine type and placement were different between the 2 makes. My Pontiac engine would not simply drop into a Chevy, Nor would a Chevy motor simply drop in to a GMC. Additional fabrication would be required.