No, you cannot retro fit a neoprene two piece seal on a 55 and earlier 235. The rear main cap is machined for a rope seal, so it’s machined a bit deeper. The neoprene seal will seat itself deeper into the cap and just leak like a sieve if you do. The 56 blocks were the first to use the neoprene seal and are machined specifically for that seal. Best gasket did not have the neoprene seal for a while but I understand they have them available.
The rope seals were notorious for leaking. It’s why our beloved motors mark their territory. But gm changed over to the two piece neoprene because they worked better at keeping oil from blowing past the rope seal. You have a 56 block so you should use the better seal. Just my opinion.
