I bought a 3/8 sending unit. Wrong Unit?

The 3/8" sender with a return line might be nice if you ever upgrade to TBI or MPI fuel injection, with an electric fuel pump. If the 5/16" rubber tube slides on the 3/8" nipple fairly easily, shouldn't be a reliability problem. You can just block off the return nipple until you need it.

On the flip side, if you plan to always stay with a carb, I would switch to a 5/16" sender. For the best of both worlds, get a 5/16" sender with return line, to allow a later FI upgrade. I recall seeing those.

Many recommend a 3/8" supply tube for fuel injection, but many production engines use 5/16" (early 90 Ford V-8's, Mopar Magnums). It seems that only Chevy V-8's and some Europeans use 3/8" (or larger). True most 5/16" ones have at least an in-tank boost pump, but I have been running an electric pump w/ TBI on my 383 and never notice a suction problem even with the pump far away in the engine bay. Fuel pours out of the open 5/16" tube so don't see how it could starve the pump.