There was quite a FordSix discussion in Feb 2022 about finding scarce FELPRO rear main seals for 200ci. I tried to find a FELPRO but no luck. I bought the Mahle JV703R because eBay vendor said it fits "200, 250, etc etc). I took a chance and lost. The Mahle JV703R is too big. But my short block machinist at Mann's Speed Shop here in Kansas City found a BOP RMS02 rear main seal. The BOP RMS02 is for "421, 428, 455 CID Pontiac V-8, and 400, 430, 455 CID Buick V-8" according to the package. I put a few photos here. Bubba showed me that it looks like FELPRO will continue to make the correct rear main seal. But they are taking a while to get in.
BOP:
BOP package
BOP vs Mahle:
BOP:
BOP package
BOP vs Mahle: