Return line fuel restriction?

I would say "yes." You are talking about the factory 1/4" tube? THAT WAS NEVER a return line and never meant for such. It was the TANK VENT on those cars that used evap controls and of course therefore limited evaporation "of the vent."

Real world: I brazed a 1/4" tube into my 67 tank when I installed a Wix return filter has an orifice for vapor lock prevention. I later used that tube for the first EFI, the little Holley TBI setup, and it FOR SURE caused some pressure restrictionin the return.

Somewhere I've read that you want the return same size or larger than supply side. Part of the reason is that it is lower pressure, and therefore hasn't as much driving force for flow