Did the rear main rope seal get changed with the mid year 235 engine?
Don't think so but I can't swear to it. It was a machining change that altered the rear main seal channel and I believe it took place for engines made in 1956 and thereafter. They didn't actually do away with the rope seal. GM still used it but the viton/neoprene/whatever rubber type seal arrived in 1956. The rope seal stopped being an acceptable product sometime in the mid 70s when they wouldn't let asbestos be used in it.
However if you want to use the rubber type seal in your engine, you can do it. First step: Very important...buy a Speedi-Sleeve number 99387. This is to restore your rear crank surface to a nice smooth and good one. If you don't do this at least have that part of your crank polished well. Otherwise you'll chew up the rubber seal quickly. Put a thin coat of Loctite Red on the crank rear piece and then install the Speedi-Sleeve carefully.
And then, all you need to do is to cut about 8 pieces of self-adhesive foil tape (the kind used in HVAC work) that are the same width of the bottom of the seal channel. Each layer of this tape will equal .004" in thickness. Apply one piece of that to the bottom of the channel of the block and trim it on each end even with the block. Install the bearing halves and set a piece of common copy paper cut 1/2 inch by about 6 inches on top of the rubber seal but not over the bearing. Just leave about 1/4" extending over the seal inside the block. Set the crank in place and see whether there is any resistance when you pull out that piece of copy paper. If not, repeat this procedure, laminating the second strip of foil in that channel and try again. If not any resistance, keep doing this until you do have some resistance. When you feel a bit of resistance (not too much) add one more strip of the self-adhesive foil and stop. That is to make up for the .004" your piece of copy paper will have added to things. Remember the number of pieces of foil you used and repeat this for the rear main bearing cap, checking the drag of the copy paper with each layer of foil. This should do as good of a job as possible.
You can see the rear part of a crank I removed from a 1956 235 a few years ago below. Good luck!
