Let me tell you about my MFI big block and one of the header swaps. The engine is a 512 B1 head making 840hp @6800. After reading everything Calvin Elston has written, I decided to build a set. the headers that were on the car were 2.25 with a 28" primary going into a 4" collector. The new headers were 2.125 to 2.25 with a OAL 26" the collector is a merge collector with 3.25 choke.
When I swapped the headers, the engine went dead lean. I had to add 10% more fuel just to get back to the same HP level. The Bsfc with the old header was .995 and with the new is 1.10. The new headers didn't make any more HP or TQ, but did show the exhaust was really working well, ordered a new camshaft with less exhaust duration and made some more tq under the curve, but that's for another time.
