Should I swap in the LS1 from my donor car or trade it for an LQ4/LQ9 6.0L when targeting about 450 rear-wheel horsepower?

At 450 at the wheels, a stock-bottom-end LS1 is past its comfort zone:

LS1 bottom end	What Speedway says it handles
Stock block and rotating assembly (anything bolted on)	400–450 hp and about 400–450 lb-ft, reliably
Forged crank, rods and pistons	600–700 hp and similar torque — suits a blower, turbo or big nitrous with proper tuning
Max displacement on a stock block	383 ci: 4.000 in stroke crank with 6.125 in rods

Speedway's 400–450 hp figure doesn't say wheel or crank. Either way, 450 rwhp means more at the crank, so you'd be looking at a forged rotating assembly or a stroker in the LS1 — or a power adder.

The 6.0 route: more displacement means more power potential for the same parts, and Speedway points to the 6.0 for big-power builds. The LS1's advantage is its aluminum block, which is lighter than an iron Vortec truck block.

How to decide: if the LS1 is healthy and the car is weight-sensitive, keep it and budget for forged internals or boost. If you want 450 rwhp from simpler parts, trading for a 6.0 gives you displacement to work with. Compare both in the donor finder.