Welcome from Austria.
It can be done but you need 73-80 fenders and radiator support for the grill and headlights to fit.
73 - 80 was the first style with the round headlights, 81 - 87 with the rectangular headlights.
https://en.wikipedia.org/wiki/Chevrolet_C/K_(third_generation)
This thread...
A lot of options here. I wouldn't go for the cheapest.
https://www.rockauto.com/en/catalog/gmc,1980,k3500,7.4l+454cid+v8,1160907,steering,steering+damper,7412
Just came to my mind: Is the pedal pushed in all the way? With a lot of slack in the cable it would not actuate the brakes as the foot pedal does not have a lot of travel.
That would explain why it doesn't move. Happens to me sometimes that I overlook the obvious.
That would be my approach: Loosen the adjuster as much as possible so that the brake cable has no tension on it. Remove the cable from the pedal. This is PITA and you will swear a lot. Step on the pedal. Pedal moves: Cable is seized or some other problem below the pedal, continue searching. No...
If everything is original I would fix the diesel and run it. Bring grandpas truck back on the road as it was when he owned it.
Having an all original truck is great imo.
Add a good water separator like mentioned before.
Post a pic of the valve. The valve can be used with or without return. When used w/o return the return ports (5/16) can be capped off. Maybe it's time to get a new valve?
I would go for it. it's nice and you just have to install it. It takes quite some time driving to junk yards searching for the parts, blast and paint them. Compared to that hours 375$ is a good price. Just my 0.02$
I drilled holes for all my emblems on my 79 K5 and had the fenders off. Measure three times before you drill. Or four. I went for mounting with screws cause I don't want them to fall off on the highway cause of aging adhesives.