If you are looking for the absolute 'best possible shape' you can think about sending it to a specialist rebuilder. If you want to keep things reasonable, then the key question will be: What does the existing fluid look like, when you drain it?? Assuming an OK on this, I would find a container large enough to hold the trans, pull any access covers, and let it soak in Diesel fuel or ATF for several days. If you see rust on a shaft at a seal you can deal with it... OTW just blow it clean, replace the covers, and install; then (of course) fill with fresh lube. AYMK this is a great time to think about your clutch, maybe re-surface the flywheel. How are you fixed for an engine support?