What's the cheapest way to get real power out of a junkyard 4.8 or 5.3 for a Challenge build: heads, cam, manifolds?

A cam first. Speedway calls a cam swap the one thing to do to a 4.8 or 5.3, and it's the biggest single bolt-on:

Upgrade	Speedway's estimated gain
Camshaft	20–40 hp
Headers	10–20 hp
Intake manifold	10–20 hp
Custom tune	10–20 hp
Cold air intake	5–15 hp
Cat-back exhaust	5–15 hp
Injectors	5–15 hp

Supporting parts: stock injectors and rails are close to their limit, so even a hot cam wants larger injectors, a bigger fuel pump and a tune. The cam needs matching valve springs.

Heads come later. Speedway's 300+ and 400+ hp 4.8 recipes both use the stock block, rotating assembly and heads, so you don't need heads to reach 400 hp.

Manifolds vs headers: headers gain 10–20 hp, but stock manifolds often don't fit a swap chassis anyway, so check what fits yours. See how much headers add.