Doh! I might have figured out the problem. I bought the clutch kit over a year ago, and this morning I looked at the receipt. The kit is for a '47 to '53 3100, which matches the truck. However, the engine is a '58 235. Can one of you experts who has swapped out the original engine for a 235 confirm that I need to use the later model 235 clutch?
Funny thing is, the '47-'53 disc, pressure plate and housing bolted right up to the 235 flywheel and transmission splines. No signs of any problems until I installed the engine.