I can tell you that my 68 Dodge R/T with a 440 and auto-trans had 3/8" fuel line from the factory. Interestingly, A12 (440 Six Pack / BBL) Roadrunners and Super Bees had 5/16" fuel lines, at least that's my experience.

1969 A12 Roadrunner
1970 Plymouth Cuda
1968 Dodge Dart