Learning experience! Unless you have not opened the packaging, I would not expect the parts store to take them back. But on the other hand, 350 parts are about the least expensive of almost any engine.
I always try to get my parts after I know what I have and what I need. STILL, I often have to make several trips to the store for every project.
I have seen engines work OK after replacing rings, bearings, gaskets and doing some valve work. I have also seen engines where doing the halfway job not work well at all. If your bores are not too worn and/or tapered and the pistons are not burned, scuffed and don"t have oversize ring grooves, reusing the old pistons might work. But it is quite a bit of work to do a "ring job", and not THAT much more work or expense to bore the block, install the proper sized pistons and rings, remove the crank and have it "miked" and if necessary have it ground undersized and reinstalled with the proper size new bearings. Something else I have experienced with small block Chevys is that they seem to wear out their camshafts worse than some other engines, at least with conventional non-roller lifters. I would ALWAYS carefully examine the camshaft and each lifter, and be very careful to put the lifters back exactly where they came from if I decided to reuse the cam and lifters. Break in lube is a must and good diesel rated oil is probably what you should use in it.
If you are just going to mess around with whatever you build with this engine, have you considered trying to find a fairly good, running Chevy V8? They made millions of them, and while I have not paid much attention lately, I always used to see GOOD used V8"s advertised all the time. Lots cheaper than putting one together. Good luck!