Just a slight addition to the main-bearing discussion: Besides the earlier 235 engines, 54 and 55 Ist *and* 2nd production I6s had the main-bearing shells pinned to the block (at least the trucks did). I have a 55 2nd 235 truck engine with the block-pinned mains. It also has shims and a 2-piece lip seal (yes, it leaks-- but not too bad). I also have a '62 261 with a 2-piece lip seal. It also leaks --but not too bad.
