Return Line Routing
AFAIK there was no return on any '66 A-body, nor did any have a 3/8 feed. Most likely someone replaced the sending & pickup unit.
Vapor lock can be minimized by using the factory routing, carb filter position, and try not to have any winter fuel in the tank when the weather is warm. I know the last can be tough since hot spells often hit us in the NE well before the change back to summer fuel.