I had the exact same problem, tried the exact same things to fix it. Then I forgot about what everyone else said and did it my way, it does not leak at all now! My sealing surface was perfect and original size.
I have a 55 235 in my 52 truck. I rebuilt it a few years ago, had a friend help me with it who had done it many times before. We put the two piece rubber seal in correctly. It leaked about a quart every week of local driving.
So I had another friend, a old mechanic and school teacher, help with the replacement of the rear main seal again, as per the instructions. No difference in the leak.
The third time I got a NOS two piece rubber seal and did it again as per the instructions. No difference.
Then I promptly pulled the engine out of the truck so I could get to it really easy and also check for a leaking cam plug, as it's impossible to tell with the bell housing on. I put a rope seal in this time, but without removing the crank, so it was not done exactly right. It leaked about two quarts in 25 miles, that sucks!
Then finally I put in another rubber seal, this time I ground the ends of the wires in the seal so it could close up better. Made some brass shim strips about 0.003" or 0.004" thick, I forget to go between the block and the seal to make the seal fit tighter on the crank. Then put it all together with black silicone behind the seal and also on the ends, just like they say not to do.
I have driven it 8,000 miles in a month, all the way around the country, with only 2 quarts added. How is that for results?
The shims may be overkill, but it did work for me. At a minimum I would use silicone behind the seal.
