dont think cheapest, parts, but rather think the best parts at the best price that you need for what you want from the engine. for instance, if you are doing a stock rebuild, then a set of replacement cast piston will do nicely. in fact if you are planning on less than 400hp, you can use cast pistons without issue as long as you control detonation. to that end getting set of TRW cast replacement pistons, or a set of silvolite cast replacement pistons on sale is the way to go.
if you are planning to try and make more than 400hp, or are planning some chemical supercharging, then you want some decent hypereutectic, or forged pistons, and again you want a decent piston at a good price. no need to pop for probe pistons when TRWs will do fine.
Is it a 302, 351W or 351C? there are tricks to getting the most power on a budget for all three of these engines. look into cam timing (degreeing the cam), cam lift, duration, and lobe centers, intake and head configurations, porting the exhaust (if you plan to use the stock heads), flat top pistons, etc. When I was learning all this stuff back in the '80s and '90s I spent a lot of time at my machine shop, and at the track, picking peolpes' brains. Most guys are very helpful. There are also, all sorts of articles on the internet which help explain all of these things, in detail, for free.
So the car came with a 302? The least expensive and easiest thing to do is to find another 302 to replace it with. When rebuilding go with flat top pistons, an E-303 or B-303 roller cam (or equivalent), port the heads, balance the rotating assembly and degree the cam.
These fairly simple and inexpensive mods coupled with a 550 CFM carb and a free flowing exhaust, (Headers and min 2.25 diameter pipes with a crossover) will return between 300 and 350 HP from a 302.