16x10 wheels fit on a 1971 K-10

[argonaut]

Won't rub on the drive train with that backspacing. But they will stick out wide. Definitely deep dish. 33" tires on a 10" wide wheel can look a little funny, like the wrong proportions. Also that really negative offset from the centerline will wear out bearings and ball joints faster as well as give you worse handling as the centerline of the wheels will be further out from the steering axis.

[Stocker]

In fact, 10" is the correct wheel width for a 12.50-width tire. You shouldn't have any fitment issues with a 4" lift. The truck might actually look better with 35/12.50s instead of 33s, but the taller tires might have minor clearance problems.

Back to your 33-12.50s on 10" wheels. I'm running 33/12.50-16.5s on 9.75 wheels, with 2.5" Softrides. No problems at all, but my front fenders were notched many years ago to clear a different wheel/tire combo (before the Softrides). Just eyeballing it, I don't think there would be any problems even if the fenders weren't notched. You should be fine with your 4" lift.

Plenty of guys run 33s on 8" wheels, which is a bit narrow but it seems to work okay. Upside is they won't stick out as far up front, and your bearings will be happier.