I'm a little fuzzy on the Clark swaps. I think there was a Clark that fit the old bolt pattern of the 47 and earlier sliding gear flywheel housing. And then there were Clarks that fit the GMC twin disc clutch housing. Whether or not there was one that fit where a SM 420 was, I just don't remember. If you look at the clutch housing and it looks like the one in a smaller truck, it will probably fit. If the clutch housing looks noticeably different than the smaller truck, it probably won't fit. Of course, there is the possibility that they used an early clutch housing and transmission, because the bolt pattern on the engine is the same on all GMC up to 302. However, if the guy who's interested is into rock crawlers, they are accustomed to doing machine work.
