Unless you are looking to put a car together in a hurry, patience is a virtue when doing a swap.
Please buy the JTR manual first and then read it through several times before jumping off into the world of swaps. If you already have a Z car, you are miles ahead. Tuning V8 engines is easy; tuning them for utmost performance cheaply is not. I suggest reading many old posts from this website and getting a good feel while digesting the information at hand. There is great literature available at any bookstore. Send for a free Summit Racing catalog if you have not already done so.
As far as what engine to buy, if you buy used, it is a crapshoot; if you buy new, it is budget constrained. That said, you have to figure out what you want (350 hp & 350 lb/ft torque) is easy for a Chevy, even a relatively stock one. Buy from friends, buy from ebay, look for deals, but be wise when handing over the cash.
Good luck!
Davy