Engine swap questions

my dream set up would be a 99+ cobra motor with a vortech

its commonly done and info should be easy to find

You can use your GT computer with a DOHC motor, you just need to get a chip burned for it. The wiring harness has to be modded a little, as will your fuel system if you decide to keep your return style setup(I did). I even had mine set up to keep my coil packs.
If you go NA or nitrous then the mach 1 motor is the way to go. You can boost the mach motor, but you can't go over 8ish PSI. The cobra motors can handle more boost due to their lower compression. However, the mach 1 motor will respond very well to nitrous becuase of it's 10.2:1 compression.