273 said

Here's a manifold/header shootout on a 300+hp 360 the difference was 1-5hp depending what rpm.

https://www.motortrend.com/how-to/mopp-0307-installing-exhaust-headers-into-300hp-crate-engine/

Horsepower​
RPM	318	360	340	SHT	1 58	tti	1 34
2500	186	185	187	189	187	189	187
3000	223	223	225	226	228	228	226
3500	269	271	272	276	280	281	279
4000	299	301	306	307	314	318	312
4500	311	313	315	321	319	326	321
4600	311	314	315	320	317	327	319
5000	303	304	308	315	312	316	311
5400	281	291	289	295	282	296	289

Click to expand...