Skidmarks,
According to the Shop Manual, the 58 block was not machined to accept a neoprene seal; that started in the 1959 blocks. I put a rope seal back in, and have been happy with the results.
Neoprene is usually preferable, but if your block is not machined for it, it would likely leak like a sieve. Check the manufacture date on the block.
3B, do you have any additional info on that Fel-Pro item?
