it seems that headers are a total dark art. The only manufacturer I have ever seen tell you the ******* DIMENSIONS of their product is Sanderson. It's like headers are a secret, if you have to ask you will never know.
With that out of the way, here is all I know:
-- A guy on another forum got Flowmasters, they are not especially beefy and high quality but they were in stock and fit on his C10 and 350 SBC just fine.
-- I highly recommend getting an aftermarket transmission support/cross-member, one with a "bat-wing" design so you can run your exhaust straight back. I got a Dirty Dingo, it's real good, my truck buddy who helps me with this stuff was impressed with the quality and he is hard to impress. Here:
-- (OTOH, it's just a bent powdercoated pipe with a couple brackets welded on, how hard can it be to get it right? But making anything of high quality and selling it and delivering it and not going bankrupt is, in fact, really effing hard, and getting harder all the time, especially dealing with the Glorious Worker's Paradise of CalifornizuelaStan.)
-- I scored a sweet set of Doug Thorley 300 Tri-y's from a guy on Craigslist for stoopid cheap, used. I have not put them on yet because I don't have a lift and my truck is 1.5" off the ground because I am such a cool low-rider dude. However these headers are so awesome and solid and sweet, I actually brought one into a bar where I was meeting my buddy, and two different girls came up and asked me about them. (I didn't convert either opportunity, both girls were little cute liberal children, one had a Prius the other her mom's Tesla, but they were definitely Tri-y-curious).
Doug Thorley, Doug's Headers, and Sanderson's are what I know and they are very high quality and rock solid.
-- Everybody says get a product with a thick flange, and spend a little more $$ for good gaskets ,and you will never regret it.
-- I asked my buddy if I should get creamic coated headers and he said,
"IDK, are you moving to effing Vancouver? Detroit? Buffalo? You live in Los Angeles, bro, rusting headers should be like #1,288 on your list of concerns. After "a$$-clown politicians", and "having to shoot and kill MS-13 gangbangers who try to rob you at gunpoint."
-- So there's that.
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Maybe your road salt snow rain geography dictates otherwise. His point: they are Auto Parts, not precious religious artifacts.
-- Take the bed off to work on your exhaust.
BRetty