46 Panel,
Hint: the distributor shaft will turn about 1/8 turn or so as you install it, so "set" your oil-pump slot about 1/8 turn "behind" the shaft position, so that when the distributor is "bottomed out", everything should line-up.
[Make sure that the engine is at TDC (valves too!) for #1 cyl (ball & pointer lined-up), and the distributor rotor pointed at the "5 o'clock" distributor cap tower, for #1 spark plug.]
Unless you've thoroughly cleaned the distributor housing, there's usually some sort of "line" on the housing neck where the black paint(or rust) stops, indicating the approx. position of the clamp.
Once you get everything "lined-up" and seated, the clamp and octane selector hold everything in place real good. Chevy used that arrangement from 1932 (?) till 1962.
(final hint: leave that clamp screw loose until the distributor is "home", then take your long screwdriver, going behind the breather pipe, and tighten it up, making sure that the octane selector is centered on "0".)
Good luck!
