Will this trans fit?????

the M5R2 was a mazda-derived transmission used behind the 3.8L factory supercharged v6, although they made a respectable amount of power for what they were.....torque on a v8 will be significantly higher (unless you have an inline then possibly the swap is worth it).... as far as it being too big, again since it was used behind a v6 in a midsized car I wouldnt think you would have any problems as far as the bulk of it.... however there may be driveline angle issues similar to a t3550 swap. biggest obstacle (other than the transmission shredding) will be finding a bellhousing to adapt a mazda-derived transmission to a ford v8 bolt pattern

You should be able to use the M5R2 bell for a 3.8, they have the same bellhousing to engine bolt pattern as the Windsors ( 289/302/351) As for it not handling the torque of a V8, then why did Ford use them behind V8's in the F series pickups?

good news then about the bellhousing.... still I would be weary of the tran's strength... sure they used them in the F series, but often transmissions have many different variants, the T5 for example had no less than 10 differnt configurations with different torque ratings for different applications