I'm getting ready to order a new tank and sending unit/pickup. My question is, what size pickup should I get? It's going in a 1963 Valiant with a decently stout carbureted 360 LA and automatic. I see 5/16, 3/8 and 1/2. Current combo is around 400 hp, I plan on fuel injection at some point and/ or possibly forced induction. Should I just go for the 1/2" and be done? Or is there a downside to that?

For your deal 3/8" should be more than enough.
I've fed a 600HP 512 street/strip car with 3/8".
1/2" is more like 650 HP stuff, as on my drag car with a Holley black pump.