Will a 4.6 fit?
 

I've seen many 5.0's and LS1's, will a 4.6 from a mustang fit? Seems like a logical way to go since the rear can also be used in the Miata.

It's too wide. It can fit but the exhaust routing gets too restrictive. I'm not sure where you're going with that idea about the differential...

-Jason

I was thinking that the Mustangs with the 4.6 had an IRS, but now that you mentioned it I realized that I was wrong and that was probably just the Cobra's

Yup, just the Cobras. Most of us end up using a differential from a T-bird (7.5).

-Jason

I am gathering parts for my 4.6 32V twin turbo build. I am pretty good with a torch & saw. It may take a few years to get all of the stuff that I need, but I am not in a hurry.

tbone, the Cobra rear will fit with the Boss Frog kit. Clearance for the exhaust is tight but the actual rear fits fine.
If you flip the factory manifolds over to face forward you may have an easier time running the exhaust but the 4.6 is a wide and tall engine. Be carefull you dont cut so much of the car that you lose structure. I might be doing a turbo set up about a year after my car is running and sorted out. I was going to do rear mount turbos by placing them where the fuel tank was and putting a cell in the trunk. Sounds easy, right?

would rather go with the ls motor or 302 , the 4.6 to 5.4 is to wide, also expensive junk yard 1200 to 1500,
you can build a nice 302 , for that,my opinion, its just to wide........