Cam & heads (ports). The oem systems flow very well and unless to get into cam/head mods or supercharging, there is little gain made with headers. Unless you go with shorty’s or actual custom built headers, 99% of all headers will require "massaging" to install (whether it's denting a tube, moving a bracket, etc.). Unless it is for racing purposes, in all reality, there are little differences in overall performance between shorties (Tri-Y), mid-length & long tube headers.....shorties actually perform excellent up to about 5500 rpm and then will have a slight drop until 6k rpm when it begins to really decline.....more than enough for street use and while some state that shorties don't work well with the new gen engines, BBK seems (and others) have different dyno results.
Hedmann in the 60's/70's an excellent product and considered one of the top mfg's with Hooker....they were built in Culver City, Ca until a few years ago. Beginning in the late 80's Hedmann quality began declining & today, they are considered an "inexpensive Header", meaning, you get what you pay for. IMHO I would place Patriot & Blackjack in the same category (although both seem to be offering a higher-quality header line also). In general, the head plate should be at least 3/8" (1/2" is best), tubing should be at least 16 gauge (14 is best).....otherwise leaks and burn-though are going to be a problem as well as the longetivity of the headers.
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Flatheads ain't so bad!
Certified backyard mechanic I & II