No need for an axle swap, you won't kill the one in there. That's a promise.
You could always find a caprice, or something on the same platform with an LT1, especially if you plan on building, it will be easier to find and cheaper than one from an fbody. I just happen to have a car for sale with an LT1 in it and a 4L60E.
Now, any engine swap is a pain in the ass. The easy way would be to find a premade wiring harness specific for the swap you want to do. As far as what engine to put it, you could do just about anything you imagine, but for simplicity sake I would go with only something that you can get a swap kit for. As nice as it is to be unique, it also means having to innovate and fabricate extensively. Along with the LT1/LS1 and T56 combo, there are also some cheaper possibilities out there. The GM 3800 (RWD configuration), the 4.3, Ford small blocks, 2.3T. Then, there's plenty of cool oddball stuff like the Lexus 4.0 V8.
Basically, take into account what your skill level is, your budget, and what you prefer.