Sounds like the tranny is a Clark 5 speed which is highly sought after.

I had the same problem installing a 302 in my 51 GMC 1 ton. I was able to use my old 228 bellhousing after finding a 6 hole 248 flywheel which is the same thickness as the 4 hole 228 flywheel. I think a 270 flywheel will work too. Then I had to order a new pressure plate at Napa for the 248 FW because the 228 PP had a different hole pattern where the PP bolted on.

Do a search under GMC flywheel and you will find a few inquiries I made and some info that should help you. One of the replies mentioned which years of 248 or 270 flywheels I should look for.

By the way, I found a used 248 flywheel at a junkyard in Windsor, Colorado. I think it cost about $80 shipped. Martin Supply & Salvage, 970-686-2460