Will LS1 headers fit a 5.3, 6.0 or LS3?

At the head, usually yes — the exhaust bolt pattern is shared. In the car or truck, it depends on the chassis the headers were built for.

Head flange: GM sells one LS header flange (part 12480130) for stock LS1, LS2, LS3, LS6, LS7 and L92 exhaust ports, noting some "may require clearancing." That covers both cathedral-port heads (LS1/LS2/LS6) and rectangle-port heads (LS3/L92/LS7), so the header bolt pattern carries across LS engines. The 5.3 truck's common 799 casting is described as identical to the 243 casting used on the LS6/LS2, and the 4.8/5.3/6.0 truck heads are Gen III/IV LS heads like the rest — see the heads guide.

Chassis is the real limit: headers are shaped around a specific vehicle's frame, steering and floor, not just the engine. LS1 headers from an F-body Camaro/Firebird are built to clear that car. Our swap guides reflect this: parts lists pick headers per vehicle (mid-length for lowered C10s, long-tubes that "tuck close to the floor" for the Chevelle, vehicle-specific kits for the E36 and SC300), and Speedway's K10 list notes that stock car exhaust manifolds don't fit, while truck manifolds do.

What to do:

• Buy headers made for your vehicle's LS swap, not for the donor engine. Start with your vehicle's swap guide.
• On LS3/L92 (rectangle-port) heads, check the port match and whether the header maker lists them — GM notes clearancing may be needed.
• Budget option: junkyard exhaust manifolds, which Speedway lists for the C10 as good to about 400 hp — again, pick ones that clear your chassis.