I think that the later engines, 87 up with the one piece rear main seal use a different flywheel. I'm not sure if it the bolt pattern or the hub that is different. I ran into this problem with installing a newer engine in an 81 Corvette. We had to use the newer flex plate. It should be the same problem with the early flywheel.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine