if you feel like pulling off the diff cover and seeing if it is. i would buy the differential unit.
some v-6 stangs had them, some rangers, aerostar vans, t-birds
check the water pump, make sure it didn't seize or lock up causing the cam to lock up. something similiar happened on the car that i originally got this motor for.
make sure you put the proper jets in and if it feels anything but smooth get it to the dyno asap so you can see your air/fuel curve and keep things safe. and watch spraying at too low an rpm, i did that, can you say fireball in the engine bay