I have an extra engine that i got originally for my truck but the motor in my mustang just up and stopped running (probably because we beat the crap out of it in our fields and in the woods) i am going to restore the mustang and i was wondering if my 300 6 would fit in the stang. the mustang is a 1987 Mustang LX with the 2.3 liter 4 cylinder.

 

With cutting and welding, any motor fits in any car. Whether it is a straight bolt-in is less likely.
Bust out the tape measure and start checking engine lengths, since you have both on-hand.
You would have to switch from the 2.3 trans to a smallblock pattern trans.

 

my guess would be no, I had an 80 with the 3.3 I6, and it barely fit. the 300 is about 6" taller, so the hood would not close.