You ask an interesting question. 56 is when the neoprene seal was introduced for the 235 in the pass car line. I truly don't know when chevrolet began using the neoprene seal for the truck 235 and 261, or when Pontiac of Canada began using them for their passenger car 261, except to say that all of my 261 blocks (1 chev and 2 pontiac) which were 59 and 60 motors used the neoprene seal.
So with your block, do you know what was originally in the block or what was in it when you took it apart? The reason I ask is that the 54-55 261 blocks are different then the later 56 and up blocks. I dont know all the differences but they had thicker bores, used milk can lifters. The 54-55 235 pass car blocks had a rear cap where the area for the rear main seal was machined deeper for the rope seal to be packed in. My assumption is that the 54-55 261 blocks were machined similarly. If you install a neoprene seal in an early motor, and it will physically install, its too loose, sinks a bit, and thus leaks like a sieve.
If Chevrolet and Pontic followed suit with the truck motors in 56 and installed neoprene seals, and I don't know that they did, then the questions becomes if the more narrow machining of the rear main cap for the neoprene seal is sufficient to accommodate a rope seal. My gut tells me yes but I really don't know and truth be told, I have not had good luck with rope seals (old style or new) and thus avoid them if at all possible. So if its the case that your rear cap is machined for a neoprene seal, I would think you probably want to go that route.
Last edited by Dragsix; Fri Apr 09 2021 03:14 PM.