Can an aftermarket under-bed gas tank work for an LS swap in a 72 C10?

Yes — it's the route Speedway took on its C10. An EFI-ready under-bed tank moves fuel out of the cab, frees up legroom and adds rear weight bias. Versions come with a ring for an in-tank pump (their choice: cooler, quieter, simple feed and return fittings) or without, for an external pump.

Pair it with a Corvette-style filter/regulator (filter, regulator and return in one unit) for mild engines up to about 450 NA hp, EFI-rated hose, and hard line for the long run to the engine. Protect every line where it could rub — fuel near hot exhaust and brakes is the thing to avoid.