I'm only able to refer specifically to late '70s J10s with 258s for my purpose as that's mainly what I've checked on. I can get all the parts at Bumper/Napa, and Farm&Fleet. Farm and Fleet has the best price, but I'm thinking of going with a custom shop bent job anyway...runs about the same there, and I'd prefer dealing there.
I did check on an exhaust for an '80 J10 with 258 a few years ago when I was looking at one...needed ALL new exhaust. I believe I could get one pipe, but not the other, so was told I'd have to get custom bent. I didn't buy the truck, so didn't need to get it. But do recall some problems with that one.
I'm not so sure on others, but I wouldn't hesitate going custom bent as it can be done, and possibly for about same price range as just the parts(if you could find 'em). Still shop around for a good place, but don't worry much on it.
'78 J10: 258, 4 speed(T18), longbed, topper
'80 J10: 360 (TFI'd), TF727, NP208, longbed, topper
1997 model American Eskimo Dog
One poodle, a few fountain pens...and too many sewing machines...