rebuilt transmission that has been sitting

There's a couple of companies you might want to check out that sell transmission parts. One is Transtar, the other is W.I.T. These are transmission parts suppliers, and I'm sure they will be able to help you out. Depending on what the end use is going to be, there are lot$ of high performance parts you can purchase. If your setup is basically stock, or just slightly warmed up, I just go with what Chrysler put in it in the first place. Raybestos waffle plates in the clutch packs, gasket and seal kit and if needed new bands and bushings, an d a shift kit. No matter what you may read, there are only a couple of companies that actually make transmission components(friction plates, steel plates, bushings, gasket sets etc). Tell one or the other of the companies I've mentioned what you intend to do with the transmission/car and they can make a kit for you with whatever parts you require.
As others have mentioned, you may not need anything other than the extension and pump seal, but until you actually crack the transmission open, you'll never know if it has or has not been rebuilt. There are lots of things you can do to a transmission externally, that will make it appear that it has been rebuilt.