Somehow I missed request for toggle switch and oil pressure switch. Just saw the stock wiring.

Suppose you could piggyback onto the 'always hot' buss in the fusebox. If there is an empty tab on the buss, remember this is unfused.
Run that wire to your toggle switch with a small inline fuse. Then toggle switch out to pressure switch, out of switch to relay, out of relay to ground. That could be 16 or 18 ga
Personally I'd keep the wirelengths short as possible and look for a spot to place the relay fairly central to the four wires connecting to it.

Only time a carb bowl should be a little low in fuel is first start of the day, week, etc.
Next time you drain fuel from a bowl into tin, set it on the work bench and see how much is there the next day.
The main thing that dries out is the intake manifold.