They always specify use of the black 3 nipple vapor separator with the 440/Hemi when using a return line. What did 383's use when incorporating a return line? Is there any reason why this separator would not work on a 383?

383's didn't use vapor separator,just a inline filter.i believe all 383 cars came with 5/16 fuel line as 44o/hemi cars were 3/8

The hemi's and 440's did not have a return line. That is a vapor return. It was there to help with vapor lock in hot weather on the RB motors. There is no need for a return line on a 383. It will serve no purpose.

Return line is 1/4 inch. Your fuel pick up may not have the return line in it. The fuel vapor separator itself has 5/16" line size both in and out so that is not a problem.