Here is an article with pictures I did about 12 years ago on this very subject.
http://67-72chevytrucks.com/vboard/s...d.php?t=122645
Long story short, I've always used small block rubber mounts and used harmonic balancer washers (big and burly) on the bottom sides of the mounts with no issues and no expensive parts to buy. If you can't lay hands on harmonic balancer washers, and big old burly washer can be substituted.