Return or returnless fuel system for an LSX/LS3 swap — which is better and where should the regulator go?

For a mild engine, returnless with a Corvette-style filter/regulator is the simple choice. Mount it near the tank, so the return back to the tank is short.

Returnless (Corvette-style filter/regulator): the regulator and filter sit in one unit near the tank, pre-regulated at 58 psi, with a short return back to the tank. Nothing returns from the fuel rail at the engine, so you save a long return line. Speedway recommends it for mild engines up to about 450 NA hp.

Beyond that (big power or boost): step up to a dedicated regulator setup. Stock LS fuel filter/regulator assembly tops out around 450 hp; beyond that, larger lines and an external regulator.