Cheapest? Without a doubt a junkyard Duraspark setup. Proven in use by literally millions of Fords. They work and work very well, plus you can get replacement parts from any parts store. Another plus is that if you want to move up to an MSD ignition box in the future, MSD sells an $8 adapter to plug their box right into the Duraspark distributor. Also Summit sells cheap advance springs that can use use to tune your mechanical advance if you like. You can also buy an adjustable vacuum advance for the distributor if you like.
There are downsides to junkyard scrounging. You may be getting bad parts, no way to really know until you try to crank the car. Distributors can have worn shaft bushings, broken advance springs, sticking advance mechanicisms, bad pickups, and leaky to the point of non-functional vacuum advances. All of which I've personally encountered. All fixable but you can just skip all that by getting a rebuilt distributor for about $65 from whatever local parts store. New control boxes are pretty cheap these days. I used to get the wiring harnesses used but lately good ones are getting hard to find. It's easier to just make one from scratch using the generic parts store crimp "spade" connectors.
One junkyard goody is the newer style ignition coil from an 87-93 Mustang V8 car. They're good for about 32K volts and very close in performance to a lot of aftermarket hi-po coils.
Parts needed, A Duraspark distributor, the cap, rotor, spark plug wires, the box, an "electronic" ignition coil (points coils don't work), and some wiring. Many how-to's on the net. Here's one good example here .
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