Here's something to watch out for as engine swaps may have happened over the years. Depending on the year model, many of the 1-ton and bigger trucks used a 10-3/4" clutch with a 9-bolt cover. The covers are no longer available and you'll have to have it rebuilt locally. You would not want to use one on a V8.
The clutch disc for both is the standard GM 10-spline but for the 6-cylinder the torsional springs are lighter than the V8. So you'll want to use a V8 disc. If by some chance, you have the 9-bolt cover, it will not work on the V8 flywheel unless you have the flywheel re-drilled. But then you'll have an obsolete clutch to deal with. So just use the V8 clutch. It'll mate with the SM420 trans just fine.
A
