Well, I've only put on three sets of headers on 67-73 A bodies.
First set, late seventies BlackJack in my 73 Duster 340, automatic, PS and A/C. Had to use a BFH to clear the power steering box and maybe other things, they leaked, and two tubes (I think) hung down below the tie rod. Still, I ran them for two years and only mildly dented the low hanging tubes. And they were worth 3/4 of a second off my quarter mile ET.
Second set, 1990 or so, another set of Black Jack or some other cheapy header, into my stepson's 73 Duster 340, manual steering, 3-speed manual. With manual steering, clearances were better, but of course still had two tubes hanging down. It only took my stepson a couple of weeks to knock a hole in one of the low hanging tubes. Patched that, and he drove it another year with those headers, beating the crap out of the low tubes. I was gonna put the stock manifolds back on, but he claimed he knew better now and would take better care of the next set of headers. So, put new ones on, with the same result as before. After one more year, he sold the car, headers leaking like a sieve.
Third set, 2010, I put TTI step headers in my 67 Barracuda 360 automatic, manual steering, no A/C. Easiest install ever, no leaks, didn't have to massage anything, and at the time my left arm was barely working due to surgery. They just fit, didn't leak, didn't hang down. And they also were worth 3/4 second in the quarter on a mostly stock 360, vs. the hi-po early 340 manifolds I had been running.
Three-four years ago, I also put TTIs in my 65 Barracuda, 340 4-speed, manual steering, no A/C. I got them to fit, but I did have to move a few things (wiring, brake lines, etc.), change clutch linkage, massage a couple of tubes, plus raise the driver's side of the motor with a later rubber biscuit for my motor mount. Much more difficult install than the 67, but they do fit and don't leak.
Never tried Doug's.