I have a '93 F250, 260K miles, 7.3 Liter Non turbo, Automatic that runs really well except for the tranny, which won't shift into overdrive. I'm thinking of getting a '94 F250, 200K miles, 6.9 Liter, Automatic (supposedly good shape), 4x4 with blown engine and transferring my engine into the '94. What do you think? Will the 7.3 Liter fit easily?

 

6.9 and 7.3l idi's are pretty much the same engine.without getting into details,the 7.3l is bored out 6.9l.
so they are a direct swap between each other.
however they didn't come in '94's.are you sure that's whats in there?
well anyway,yes your 7.3l will go into the '94.it had a 7.3l from the factory after all.
the '94's a more solid truck i take it? thus the reason for the engine swap,rather than trans?
.....ahh wait,the '94's a 4x4.

in '94 there were two engines,the 7.3l idi (like your '93) and the first generation 7.3l powerstroke.
if it has a 6.9l idi in there,then it's likely an original 7.3l idi truck.

at any rate.slap your 7.3l idi in there and go 4 wheel driving!