Engine swap
 

How hard would it be to swap an 05+ GT engine into a foxbody? What all else would need to be changed?

For one, you will need adaptors for the mounts and also, an aftermarket k-member designed for mod engines, and the 05 computer. Also, you will need a trans to go along or a new bellhousing for the T5. I'm sure there's other bits that I missed but it's no different from swapping a 96-04 4.6 in, just for the 05s+, you need adaptor plates for the mounts.