No for multiple reasons. First, the bell housing on the 300 is not the same as the 4.6, so it won't bolt up to the block. The M5R2 did come behind the 4.6 in the later F150s, but all the truck transmissions have the shifter as part of the top plate, which would put the shifter about 18" too far forward, which is not an amount that you can overcome with creative linkage. In theory you could use the tailshaft housing off an SC M5R2, but the output shaft is different between both transmissions, so the only way it could be done would be to have a complete SC M5R2 and a complete F150 M5R2, and swap all the SC internals, output shaft, and tailshaft onto the 4.6 F150 case.
Alternatively, you can get a TR3650 out of a 05-10 Mustang GT, a custom driveshaft, and a speedcal, and it is a truly bolt-in swap.
Alternatively again, you can use a T45 from a 96-98 Mustang GT and have to do some fabrication, but that would be the cheapest way to get a 5-speed in a 4.6 MN12.
Last but not least, you could hunt around for an ultra-rare 91 Lincoln Town Car 4.6 engine block, which has the small block Ford bell housing,and that would let you swap a complete SC M5R2 behind a 4.6.
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