The problem with the Best rope seal is it is not like the original rope seals that were used by GM and the aftermarket. Unfortunately, there are not any better new replacements. If you can find an original type seal (they used asbestos as a component) that is still pliable, I think they are the better call but finding decent asbestos based rope seals is getting harder all the time. The current crop of seals is fiberglass based and will work but it appears not as durable or effective as the original.
The neoprene seal was introduced for the 56 model year. The announcement can be found in the shop manual. So you can find a block date coded 55 that will use the neoprene seal but it will be for the 56 car and truck line. I have a 55 dated block, September 55, with the neoprene seal so that is 56 model year. The June 55 block I have is rope and shims for 55. The June block is machined differently then the September 55 block at the rear main cap. On the June block its cut deeper to accommodate the rope seal. You can physically get the neoprene seal in but its not a firm fit, sinks and separates and then leaks.
Over the years I have tried a couple of things to see if there is a a way to use the neoprene seal, spacers, cut down rope seal as a packing so to speak, a piece of thin spring seal to impart pressure, none of it worked and I got tired of pulling motors and taking them apart.
So short of swapping over to a 56 and up block, you will have to see if you can get the main cap off, sneak the upper seal out and fish a new seal in, and replace the old seal in the lower cap. I myself have never been very successful in getting an in car re seal completed and have it work. that is not to say you can't. Just not me. We have had a couple of threads on this issue, search around because I know a couple of the fellas here have done it successfully.
