Well the doughnut gasket would be on the outward end of your exhaust manifold connecting to the exhaust pipe. If you have one - definately change it out. The popping is most likely coming from where the exhaust manifold mates to the block. Regardless, check all the connections. My 66 has the inline 6 so my headers will be different than yours. I had no clearance problems as I don't have air conditioning and there is nothing else in the engine bay that will obstruct headers. If your car is lowered a bit I don't know how much clearance you have but I would think there would be no problem with shorties. Just spray all the bolts with WD40 and back them out a tad and then tighten them. If you are thinking of aftermarket headers I'm sure there is someone here that can direct you in the right direction. Keep us posted!
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
30 Years & 30 inches apart in the driveway!