If you are trying to stay cheap, ignore me when I tell you to get a T56 6 speed from an LT1 car tongue.gif
Seriously though, a T56 would be your best bet, but from what I hear, it would run you about a grand to pick one up. Keeping that in mind...
The one and ONLY thing an auto is good for is drag racing. Even then, if you are truly a good driver, you can make the difference null with a good 4 speed. If you don't care about banging off 10 second or faster passes on the strip, GET A MANUAL. Trust me. They will handle far more abuse than an auto, and they are more fun to drive.
If you do decide to go auto, the 700R4 really is a good auto tranny for a car that's all stock. Mad Malibu just swapped out his turbo 350 for a *built* 700r4 due to his low rear end gears, and his wanting to drive it on the street. It just wasn't possible to cruise above 55 in it with the TH350. However, with reasonable rear end gears, a TH350 can be a good cruising tranny, just realize that your gas milage will suffer due to the lack of overdrive. It is the strongest auto than you can get, short of the newer electronically controlled ones. Malibu Stacey can tell you what all is required to swap the autos, I know that he had to use a different driveshaft, and he ran into other problems. Auto's are a nightmare.
Having said that...
Get a T5. It is a 5 speed manual that can be found in junkyards with 3rd gen f-bodies. Depending on the year, they came only with certain packages. I know that towards the end of the 80's, you could not get them with the 350 motor, only the 305 RS. I don't know shit about the V6 cars. You may have to search for a while to find a manual car.
In order to due the swap, you will need to get the driveshaft from the donor car, along with the pedals, and possibly the center console. If it's a hydraulic clutch (i'm unsure of this), you will need to get the clutch master cylender, slave cylender, and the line between. Make sure you get the bellhousing too, with the tranny. If it is a manual clutch (no hydraulics), just be sure to get all of the linkages.
Good luck.