By way of a welcome, I should warn you that some of my opinions probably go against the grain.
Headers are something a lot of people rush towards. They will do 9/10 of squat on a standard motor, with the 10% benefit
mainly being in looks. However, if the exhaust manifold is cracked or damaged and you're anal enough to want to "do things right" sometimes it's cost-effective to fit a header to buffer against other (planned) mods. But don't go bolting on a set and expecting miracles.
You probably have the Load-o-matic distributor. I dislike this because it's designed to work with a stock motor in new-ish condition. Forty years after manufacture of the car, how many fit that criteria? I dislike add-on modules such as Crane, Pertronix, Optospark and others sell; they rely on function of the distributor being perfect otherwise (see prior remark)...
So - a more modern distributor is a Good Thing. A regular Ford Duraspark with a
genuine GM HEI module and appropriate coil, will give plenty of spark and not cost a bomb. While the MSD or Crane Hi-6 are nice to have, for people to install one and then say how much better the car runs, makes me suspect they weren't in perfect tune before - and the new ignition is
masking another condition.
Even if your car has the smaller distributor shaft it is easy to machine this aspect of a Duraspark distributor to suit. You need to also co-install another oil pump and pump drive shaft, so the oil pan comes off. No big deal but a good time to look at the bottom of your bores and inspect clearances. Remember that the "rebuilt" Duraspark you buy will probably be mis-curved and wrongly weighted...
At this point, you are free to play with the carb. Without pulling the head off, you'll be bound to work with a couple of different brand carb adapters, or a selection of slightly later year carbies. A smaller synchronous 2-barrel may work well in your instance with an adaptor. With a few years wear on a 170 and stock internals it's pretty much all over near 4000 RPM, so you don't need to worry about air requirements at 5500... It's more an issue of keeping the venturis sized right for good atomisation, and tailoring the fuel curve in conjunction with ignition to avoid flat spots or other mishaps.
Consider a couple of gauges to add: Fuel pressure between carb and pump, and manifold vacuum. These needn't cost a bundle, and will eliminate a whole lot of speculation! Timing chains stretch badly and simple renewal of the stock chain will often benefit the motor.
Unless compression test results are horrific I'd urge to leave the head alone and attached! You'll gain a few points of "oomph" by working through the above and it needn't cost the earth.
Regards, Adam.
