The 235 vac advance canister is getting hard to find. And you do have to remove the dist to change it, as it clamps around the base of the dist. The vac advance can be removed on the newer engines (230-250, etc) w/o removing distributor, but not on a 235.
Johnnys right, the cam turns 1/2 as fast as the crank, and the dist turns the same speed as the cam. Ergo, the dist turns 1/2 as fast as the crankshaft.
Really want yer mind blown? The cam in a I-6 chevy turns opposite crank rotation, yet the distributor still turns clockwise like a V8. The trick? Reverse cut distributor drive gears.
chip
