'71 318 Barracuda - manifolds, my first car, did everythign I could to kill it and squash horsepower during my steep learning curve.
'71 340 Cuda conv.- manifolds, basic full resto anyway, wanted to keep stock look.
'74 318/340 Barracuda - headers, 15/8 cheapie hedmans. No dimpling, but flattened tubes repeatedly on several sets while owning/driving.
'70 383/440 Baracuda - removed manifolds, still runs Casler 1 7/8 primary, 3" collectore. Prices are less, no dimpling, trans can be removed without touching headers, now 10 years old, no rust holes, no coating.
'70 383 RT - headers, 1 3/4 Hedmans, no dinging needed to fit, clears bell housing. SO good at 5 yras old they got coated. Now 8 years old.
'71 440 Cuda - headers, same Caslers as mine, bought 3 years later. Driven daily in summers, 8 years old, no coating.
'71 440 Cuda - headers, Hooker Competitions, 1 7/8 primary, coated, no dinging required, 4sp, P/S
'73 wedge Barracuda - headers, TTi, 2" primary, cannot remove transmission without removing one header, hangs lower than my Caslers, coated, cost 3 times Caslers.
I also have run headers and manifolds on my A bodies, manifolds on my C bodies, and headers on my trucks.
Conclusions found over the years: Manifolds run cooler, are quieter, fit much easier, make maintenance much easier, make custom exh more expensive, can be initially expensive if you have to buy the right ones, do sacrifice low end trq, and upper rpm power. They can make power, but every engine, bar none, with any cam or head design, will make more power with a properly sized header as opposed to any available factory manifold.
Headers are like almost anything else. In TTi's case, unless the cylinder head type and chassis force me to, I'll look elsewhere. They are too expensive for what you can get if you shop around. Headers fitting is a relative tive. You wont know until you try. I have one set that fit really terrible, a couple sets that fit with nothing else needed. The rest may have needed a tweak here or there. Nothing huge. The thinner flanges will be used up much faster than a quality set with 1/4" or 5/16" thick flanges. I've never had an uncoated set rust thru, unless it was a plow truck that sits all summer, and most of the winter. I generally buy plain painted ones for that reason. A "show" car should have coated ones, to impress everyone. With proper looming, boot choice, and care, wires and boots will last for years with them. I rotate wires off my Cuda, onto my weekend dump-run swap-meet truck. Leaks occur from crud on the flanges of the heads, too much engine movement, and impacts with objects. Sometimes the coating process leaves "hangers" too. Flanges can be bent from over tightening. You're better starting lightly torqued, and going back over them a few times then socking them down. Proper exhaust hanger use can make headers seal forever. Most palces I know consider the header flange and engien an exh hanger. If only one other is used, you're asking for failure. Also, if the car is strong, a tight exh will act like a torque strap, and that too will spit out gaskets. If you make serious power, tie the engine down better.