The 3sgte conversion has been done many times before. I can't comment on how hard it is, but I wouldn't say its insanely hard. The motor itself will fit in the bay, but new mounts have to be fabricated. Check this thread for more info:
For around $8k to $12k you could have one running in your car. A 4agze conversion could cost around $3k to $4k if you do it yourself, including cost of halfcut.
Whichever conversion you do, make sure you have spare cash, because things can go wrong and you'll be stuck without a car.