Hey, I have some info about this one!
My 50 was a 2 ton dump truck. We bought the truck from the cab forward, and disassembled it and junked the original frame.
In the begining, we attempted to put this cab on a 77 1/2 ton long box 2 wd pickup frame. The nose of the 50 is 6" to 8" shorter then the 77, that puts the firewall 6" forward. Also, the relationship between the front tire centerline and the radiator position is also shorter on the 50 compaired to the 77 frame. The cab is also shorter on the 50. The lower you go on the frame, the worst the porportions get.
If you cut the frame horns off the 77 frame and shift the nose of the 50 towards the rear a couple inches, with smaller front tires (15" wheels) they can still be fit into the front portion of the 50 fenders. You will have to modify the fenders and wheel arches to look right with the smaller tire/wheel combo. Then you need to cut a hole in the firewall as wide as the whatever engine your using and set it back into the cab about 6". That takes up a lot of floor space, puts the new firewall at the back edge of the dash, the width of your engine clearance cut. The rear cab mounts can be moved forward on the 77 frame to match up with the 50 cab, and the box/bed will sit 6" more forward then it did with the 77 cab. Also expect to do some modification to the floor pan of the cab, originally the 50s cab floors are nearly flat, because the engine/trans sat low. The 77 frames drop under the cab but the 50s frames were straight, to get that 50 cab low, you will have to cut the floor under the seat and at the firewall to get down to the frame then build a trans tunnel to clear the trans. I think I still have some poor pictures showing us trying to shift the cab and front clip around so we didn't have to cut the firewall. Then a few more pictures after we cut out the firewall and most of the floor, in an attempt to lower the truck. Really ugly, and not a project for the weak at heart.
I ended up putting that 50 cab and clip on an 80 Dodge 4x4 short box frame. I raised the cab 5" off the frame, and welded the firewall back in. The engine (rear of the valve covers and sb dist) sits under the cab a couple inches. I ended up making front fenders from just below the top curve all the way to the bottom and narrowing and rearching the fender extensions. I also moved the wheel well openings 3" rearward. I filled the gap under the cab with running boards. My pickup box is hand fabricated box sides with the 50 rear fenders and the front of the 50 box narrowed 2". The tailgate is home made and the box floor is 1/8" steel. The cab is bassically stock except for the dash, which was home made because the original dash and guages were all rusted junk. The seats were whatever was on hand at the time. This truck has been together for 7 years and just got a fresh 360 last spring. The frame and drive train are standard issue 80 1/2 ton 4x4 stuff, 360 4v, 727, full time transfercase, 9 1/4 rear suregrip, Dana 44 front both with 3:55 gears.
