If using a high flow,volume electric pump I would use a 3/8 supply,3/8 return, that is how my car is, 5/16 is ok for EFI, because it runs a much higher pressure, but for a carb,3/8 would be better for a return, mine is carb for now.

1968 Charger COLD A/C Hilborn EFI
512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim
2020 T/A 392 Stock 11.79 @ 114.5