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The doors are easy to modify because the inner structure doesn't need any work, and they are usually in good shape. The cabs are basically the same except for the cutouts in the doors and floorboard for the front fenders.... A friend of mine and I put a complete C50 floor in his K10 a few years ago. We had to modify the front floor where the pinch weld is but it was pretty easy.
I can guarantee the floor pans are the same except for the front corners, transmission hump, and where the trans hump meets the front toe board, and those areas aren't that hard to massage....
Here is the thread where we put the C50 floor pan in the K10:
http://67-72chevytrucks.com/vboard/s...d.php?t=490272
Here is a thread on some of the firewall area differences in the C50 and C10/20....
http://67-72chevytrucks.com/vboard/s...d.php?t=657508
As far as the cluster all you have to do is trim the plastic up to the next line where it fits around the steering column. That cluster should also have the 80# oil pressure gauge and maybe a Vacuum or air pressure gauge.
LockDoc