[olddog]

I just dropped my Mallory distributor into my 5.0 block engine yesterday. The distributor has an O-ring just under the flange that sits on the block (clamp goes over it). The distributor housing is slightly smaller than the block, but the O-ring makes a tight seal. No wobble or looseness at all.

As for the gear, some roller cams use a steel gear. You just have to make sure the gear is what the cam calls for.