You've heard it before ?

First off, nice score on the 68 Valiant. I'm jealous, while keeping an eye out for the same car. I agree with all the above advice. Making a master plan is important, what are your goals for the car? With a slant, balancing complementary parts is a huge part of improving performance as opposed to throwing parts at the project. Doing the HEI ignition and recurving your distributor are a good start. Living in MA, unless you only drive this on only your hot summer days you will regret not having any intake manifold heat. Does your Clifford manifold have the water jacket? A far better route for a street driven car than headers are Dutra Duals. Yes, they are now more expensive since they were bought out by Aussie Speed, but they really work and are worth the coin. If you don't want to get into the internals, you are wasting your time and money with any 4-barrel carb. You need a bigger cam, more compression, and head work to get any benefit from a 4-barrel. The 2-barrel Holley is a better way to go (or a Carter BBD, or a Ford 2100). For exhaust, on any slant full duals are not necessary. If you want them because you like them, then fine. But it's extra weight that won't improve your performance a bit. A single exhaust properly set up will be better and cheaper.