Try to find a running engine that's had some care. For example, we bought a 20k mile L96 6.0 for my brother's 78 Z28. If you don't mind finding an unwelcome surprise or having to pull the engine later to fix it, then by all means get one at Pick-Your-Part or some other source where you don't really know what you bought until you get home with it.
A cam and valve spring change will get you to your power goals.
If you want an automatic transmission, I recommend a 4L80E. The 700R/60E transmissions are marginal at stock power levels, in my experience. Spend the money on a quality (Circle D, FTI, etc.) torque converter with a stall speed to match your combination and how you want the car to perform.
There are a few options if you want a manual transmission but they are all more expensive. Post back if this is how you want to go. The T56 Magnum or the new TKX would be excellent.
Rear gearing depends on your tire size, transmission gearing and how you want the car to perform. With overdrive you'll likely be in the 3.23-3.73 range. With a three speed (why?) You'll probably want to be in the 3.08-3.23 range.
If you're interested in what my brother and I did with his 6.0, check the Camaro thread linked in my autosignature. That starts with a lot of body work so you'll have to go a few pages in.
If you'd like to see my experience with the 4L65E vs. 4L85E, check out my GMT900 thread where I swap to a 4L85E.
There are lots and lots of ways to skin this cat. Once you make a few decisions and have a few parts of the swap purchased, the others will start to fall into place.
Hope this helps.