RE: pipe resistance
The o-rings for normal 7/8" No Leak flanges is a #212 o-ring which has the dimensions are 7/8" I.D. 1-1/8" O.D. with a 1/8" cross section.
The white teflons do make for a tighter crush initially but with only a little run time the header embeds itself in the teflon because teflon has like Zero resilience then it looses the tightness it had and starts to bypass exhaust. Best is both Viton and the cover cut enough that it smashes the o-rings into every nook and crany and is especially tight. Once the o-ring seeps one iota of exhaust the fire passing tbe o-ring just burns it up.