As you look to being going down the GM engine route (ref your comment regarding a Z28 Camaro) you could easily get hold of a mid 80s / early 90s Camaro / Firebird and liberate the engine and autobox for your build.
I've purposely ignored the later cars as the autoboxs have an interface with the car's computer and would require some electrical modifications.
Starting from the early GM Turbo Hydramatics - TH400 and TH350 are 3 speed autoboxs with a direct 1:1 top gear. These boxes are bomb proof but physically bigger than a contemporary manual box. I have a TH350 in my Dax.
Next we have the TH700R4 and TH200-4R. These are 4 speed autos with overdrive on 4th and a lockup clutch in 4th. These boxes can be found in cars as noted above.
Then came the computer controlled 4 speed autos with overdrive and a lockup clutch (basically a refinement of the earlier 4 speeds) 4L60E and 4L80E, you would have had one of these in your 1999 Camaro.
The TH200-4R is fairly compact but is limited in it's power handling capabilites. However, if you use a mild Chevrolet 305 or 350 V8, a TH200-4R would be adequate.
TCI build uprated versions of these
http://www.tciauto.com/gm/gm_2004r.htm
Steve