please do tell ive only done a few rebuilds so always learning
[low compression parts] So what happens is sometimes the replacement pistons are a little shorter and don't come to the top of the block, resulting in more space between the head/valves and the top of the piston when its at tdc. That accounts for the denominator in the compression equation... when you divide by a larger number, the result is smaller.... thus less compression.
You can try to compensate for that problem by decking the block, shaving the heads, and using a thin head gasket.... but its less expensive to get the right pistons. Unless you need to deck and shave to correct the old iron.
I think if budget is limited, it is better to spend on the heads/intake/cam rather than increasing displacement. Unless you don't like huge chunks of aluminum under your hood.
Another thought if budget is limited. Measure your 383 bores, see if you can use standard pistons. Then find a couple set of good used ones from a pre-71 block. You could research the compression ratio the factory rated them as, and get pistons from that vintage. You might even find someone with good used rods, but be careful as sometimes magnafluxing and resizing can cost as much as new ones.