More than half the battle is finding a good, unmolested core these days. Used to be there were unrebuilt, original TH350 trannys for the asking. Now it seems every last one of them has been "rebuilt" by some mouth breathing knuckle dragger before you ever get to lay hands on it.
I wouldn't buy a core I couldn't personally inspect. I'd suggest craigslist or the local salvage yards, rather than eBay.