Best bang for the buck will be just cleaning it up and running it. Dont go changing rings and bearings. If it turns over freely now, we can postulate it will run ok. In other words, these rings and bearings are seated and should work fine now unless a bearing is spun and your screwed anyway. Sounds like you have a perfect candidate for just bolting on a few things. Compression ratio will suck, but changing pistons is money and you will end up figuring out it has some taper to it and want to bore eventually. Then standard hi comp pistons are garbage to you.

Another member recently tore down and engine and spent some money, on a budget, then had to spend more since you basically cannot just tear into one and piece meal the thing. You might find that thread, i think it was 71adam440 or something like that.

Pick your cam and a good timing set. Then look to the heads, maybe do a valve job or guides. You could find a set of 915 heads sure, spend money on them, then trash them later when you decide on a full rebuild and aluminum heads. That's just cash burned for a small improvement.

If you go the low buck route(many people have done these), plenty of budget becomes available for other parts of the truck. You'll soon find things to spend on. Or buy new rings and bearings now and buy them again for the full rebuild, which could be as soon as something goes awry doing a rering and bearing job.