I'm "less gooder" (as my 7-year old daughter would say) with the '50s stuff than the '40s stuff, but . . .
The starter, me thinks, grounds through its case and on through the engine.
The ground straps you need are: Battery negative to frame (on the '40s trucks, this is a braided, ribbon-style cable, going from the Bat. negative post to a bolt on the transmission corner), and . . .
Engine to Frame. This assumes the tranny-engine combo is cleanly isolated from the frame, which is unlikely in any real-life scenario (your carb. cables are a good example of one path the electrons can take to get from the starter, through the engine, carb, cables, dash, frame, for example). BUT, common wisdom is that a good-sized, dedicated engine ground strap to the frame is necessary. Don't think it matters where, as long as the connections are good and clean. Be sure to wire brush the paint away from the frame at the connecting point.
The little electrons will take the path of least resistance, and bigger cable means less resistance. If you have this, maybe it won't ground through the carb cables!!!
Hope I'm right, and that this helps!
-Michael
