If you use the Mazda tranny do some research. It has some rubber/nylon/plastic whatever it is plugs that like to fall out and it loses tranny fluid (and yeah I believe it uses ATF as well) and well fail.
I believe I've seen them put in steel plugs and keep it topped up and they hold up ok. (This is per a lot of discussion about using them behind Cummins 4bt diesels so they can take the power no problem.)
You can also use a ZF5 but I believe it has a pretty low 1st gear (not quite granny gear but close), the Mazda has a pretty low first as well.
You can put a world class T5 in there (after 93' from a 5.0 302), I couldn't find the info tonight, tired and just found snippets but essentially it'll fit since it was behind a small block. You don't want to hammer on a T5 though and it can't tow (which if its in a rat isn't a huge problem unless its a truck and your gonna use it like a truck) the torque is what may kill it if you get a bit to happy with it and the 300 has that in abundance. Thats debatable though, many may not think the 6 would kill it. It does have about the nicest ratios though as its more of a car transmission.
And of course there's the top loader, but they're pricey and getting harder to find.
Course for automatics the c6 will take anything the 6 can throw at it, and the E0d is basically the same with a OD, not sure if it uses a computer or not as I haven't messed with them before.
Just some things off the top of my head and of course you'd want to research all those options first. But that 300 as mentioned is probably one of the best motors Ford ever built and 200-300k miles have been seen on them no problem, its a brute. Darn thing's torque is at like 1600 rpms. Good score, heck yeah I'd use one in a minute.
Regards,
D.