I can't comment on mounting later I6s in these trucks, but I can tell you my experience with them.
While in college I had a 1964 Chevy II wagon with a 194 that I replaced with a healthy 250. They are externally the same, like a 283 to a 350 or 400 sbc. I built it with a 194 head for higher compression and upgraded the valves to 1.94 intake and 1.50 exhaust, and shaved the head and block to 0 deck. Compression was about 12.5:1. I had a cam from Clifford and a set of dual headers and glasspacks, with a homebuilt intake system that used 3 stock 1 bbl carbs. They were not connected to each other, but simply 1 per intake port. It was a bear to get it all tuned. It ran great and surprised quite a few local V8 guys, but eventually broke a timing gear and was relegated to the back yard by the (now) ex-wife.
Devin
