I would just get an SM420, that came with them when new. They are fairly easy to find and you will have no exotic problems getting the truck back on the road. You need to go through the casting number search to pinpoint what engine you have first. That is an easy search. Depending on your exact truck, the rear end upgrade, new gears, may be the easiest route to a little more speed in high gear.
If you have two engines that spin, do compression checks and possibly start them on a stand and see if they are any good. Everyone here seems to like the leak down tests as well. The engines will both most likely need some internal cleaning. My guess is that both are going to run and would work. I would tend to put the GMC back into the GMC Truck if possible.
I would also guess you have Huck Brakes. They too will work fine but require more care and maintenance than newer brakes. You just have to learn about them.
You have a lot of options.
Thanks for the info. Both engines are GMCs. I have the manual transmission in the 52 as well as a spare one that came with the spare motor. They must both be what you are talking about.
I have heard about kits for swapping out the rear gearing to something a little better for modern driving but so far I have not found out who is selling them.
I have no clue about the brakes other than that they are drums. I am looking into the brake conversion kits but that isn't a priority yet. I am keeping the straight axle on the 50. (A dakota IFS kit will go on the 52)
