64 Dart Fuel tank sending unit question

Another question for you wonderful dart experts! We are replacing the fuel tank on our 64 dart gt, so we also want to replace the sending unit. I find the part on-line, but it comes in two sizes. I am certain ours has never been replaced (considering the condition of the car), so what size outlet was original sending unit on the 64? "3/8" with or without "fuel return"? or "5/16 outlet? Our repair manual doesn't state. Thank you guys (and gals)

I called a parts store to inquire about why a certain valve seat was considered "premium", like what was the difference in material from the "regular" valve seats, the gentleman said he had no idea what they were! -never thought I would learn this much about cars!
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