Fuel sending unit ports and Driveshaft

[Jay78k20]

Hey yall, I’ve got a 1978 k20 custom deluxe with a 350. Got it with a 4 speed manual but switched it out for a 3 speed automatic for a racing shifter. So far I’ve ran into 2 major problems. #1 is to do with the fuel sending unit. We replaced the og 2 port with another 2 port unit, however it’s been so long since we pulled it, we don’t remember which port is the sending one and which is the return/vapor line. For reference, it had the dual saddle tanks, but we dropped the drivers side completely and are only running the passenger side. My second issue is which my driveshaft. We got a correct length one, however the slip join is at the rear differential. It looks like a front driveshaft just longer with that slip joint at the differential. Everyone I’ve talked to already have never seen that joint at the back and only up at the transmission side. If I could get any answer to either or of these questions I’ll greatly appreciate it. I’ll try my best to provide any extra information if needed but forgive me as this is the first truck I’ve fully dug into. Thank you in advanced. See pictures for better explanations:

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[idahovette]

On the drive shaft, the slip at the rear is the CORRECT way. My 75 K20 has been that way since I bought it new

 

[Old Guy Bill]

Yep, my ‘78 K20 has the slip yoke in the rear also.
That’s a good pic of the original heat shield for the fuel lines discussed in another post also.