I know this thread is kinda old, but I figured I'd throw some more info out there if anyone is considering it. The Mark VIIIs make nearly the same power out of the box as a 96-98 Cobra, the main difference being the method of controlling the IMRCs and lack of a forged crank. Cobra motors are also hand assembled, whereas Intechs (the name for the Lincoln 4Vs) are assembly line stuff. 99 and 01 Cobras make more power than the other NA 4Vs, excluding the 03-04 Mach 1s. Mark VIIIs can be had dirt cheap (I've seen whole cars selling for $500), Cobra motors are typically much more expensive. T45s would probably be fine in a Z, make sure you get an aftermarket shifter with built in bump stops and set them properly. The #1 reason all Mustang transmissions (T5, T45 and 3650s) fail is missing gears or slamming into said gears far too hard. I have a 96 Snake, if anybody needs measurements on anything or needs a few pics of a stock setup let me know and I'll do what I can to help.
Quick 4.6 food chain for reference
Mach 1------> 99/01 Cobra-----> 96-98 Cobra-----> Lincoln InTech