As a brand; Headman Headers has consistently offered the best fit. Hooker has offered the highest performance headers for drag racing, Jere Stahl offered his own take on equal length headers that were more compact, but never caught on due to their "funny" design of four tubes merging into two tubes that exited with one tube called "Tri-Y" headers. Headman offered quality and ease of installation, but made no claims other than thicker tubes and thicker mounting plates so that they took longer to rust out.
In every case read the foot notes (in the back of the header catalog, or on a separate page on the header manufacture's web site) because the big print giveth ("these headers fit every Chevy ever made with any motor), but the small print taketh away (foot notes says except for: and lists 36 lines of exceptions). If you have an exception they will not fit unless you modify them. Headers are for stock cars: built just the way Chevrolet screwed them together fifty or more years ago without any modifications to port location or height. Bolt on aftermarket heads, a different power steering pump or different A/C compressor or hydraulic clutch instead of a mechanical one, or a bigger than stock transmission (such as a 4L80 replacing a PowerGlide), and they will not fit.
Because of this I have learned that it costs as much in time and aggravation to just build a custom set of headers from a box full of header parts. Actually I don't build them I let a muffler shop build them for me just the way it was back in the Sixties when Jere Stahl, Doug Thorley, Bob Headman, and Gary Hooker all sold there tubes out of a muffler shop: because cutting fitting and welding thin sheet metal tubes requires experience and welding knowledge I don't possess.