What fuel pump and tank should I get for a third-gen that sat for 20+ years — and is a 255-lph pump overkill for a stock engine?

A 255 LPH pump isn't overkill. It's a common, budget-friendly choice — Speedway puts one in its universal in-tank module and rates it for about 500 hp. It'll feed a stock LS easily with headroom for later mods. Size yours with the fuel system calculator.

On a car that sat 20+ years: we don't have a source that covers cleaning an old tank, so we can't give a specific recommendation there. A new or EFI-ready tank avoids the unknowns, and an in-tank pump is cooled and quieted by the fuel around it.

Lines and hose: an LS needs about 58 psi. LS EFI needs about 58 psi vs 5–7 psi for a carbureted small block; parts-store hose clamps/rubber hose are not adequate at 60 psi. Use AN hardlines, push-on hose/fittings, or ethanol-safe hose (e.g., Earl's Vapor Guard).