Will these headers fit with my stock exhaust manifolds still on?

No — these are a direct replacement for factory manifolds. You'll need to remove the originals first. The shorty design is specifically engineered to avoid suspension and steering hits, so once you swap them in, clearance shouldn't be an issue.

Do I need to use gasket sealer or high-temp paint before installation?

Gasket sealer at the block mating surface is standard practice for any header swap — grab some copper-based high-temp product at install time. Stainless steel doesn't require coating to resist corrosion, but some builders wrap or paint for heat management if their engine bay gets tight.

What fasteners do I need to bolt these down?

You'll use your engine's factory bolt holes. Grab studs or bolts rated for high-heat exhaust work — most builders go with stainless or mild steel hardware in the appropriate size for your big block. Check your service manual for torque specs and thread pitch before ordering hardware.