i had dougs three tube headers on a 4 speed 69 firebird back in the early eighties . i had to take a hammer and beat the tubes to adjust for clearance on both sides . they hung low underneath the car , bolts on the block would vibrate loose . i would have to tighten them once a month. i was not happy with them. unless i was using my bird for racing events they are not for me i will stay with the pontiac manifolds. a lot less head aches .