Eric is correct about the '58 to '62 forged rods (commonly known as rubber rods). In my opinion, cast rods are stronger. I used a set once and one broke at 5800 rpm, and of the rest, three were bent. As far as four bolt vs two bolt mains, unless you are going all-out racing, two bolt will be fine. It would be advantagous to switch to studs instead of bolts, though.