Use a bulkhead fitting to either go thru teh sender as mentioned or into a suitable flat spot on teh tank. Make sure the location allows you wrench access and make sure to use sealing washers - nylon, copper and a bonded-rubber are available. Watch what type of bulkhead fitting you get, as some only permit connections on one side, not both.
If going on the sending unit, make sure to choose your drilling location based on teh size of the nut vs other items - a sending unit isn't as big as you imagine when trying to fit a nut on the inside (not much room for drilling error).
If the returned fuel returns below fuel level it will have higher back pressure and it will vary with how full the tank is. Returning it above fuel level makes more aeration of it and higher vapor pressure in teh tank. It's a tradeoff IMHO.
If trying to assemble a home-brewed fitting from brass hardware-store items, consider that they are sometimes smaller in ID compared to a JIC fitting.
Try to point the returned fuel *away* from the pickup – returned fuel will be aerated and you don’t want that heading right back into teh pump.
My method is a BH fitting, with a -6 AN female hosebarb toward the engine, and a suitable piece of bent tubing (with a -6 nut and sleeve flared on) into the tank. If you work it right the bent tube route quite far from the pickup as it can be installed and tightened as you feed the sending unit thru its hole.
EDIT:
Mcmaster.com has this BH fitting in -6 but it's $30:
http://www.mcmaster.com/#50715K582A hydraulic supply house would have steel fittings much cheaper.
Use your imagination, see what's available, and try not to get cross-eyed looking at tube, pipe, JIC and SAE thread types!
