Er, how often do 8A80s become available? NOT a good idea. Vastly better idea would be to put an LS1 in there which would be more compact and significantly lighter. And capable of probably double the power.
DON'T use a 6G74. Use a 6G75. Capable of a lot more power in atmo form than the 6G74 thanks to more aggressive cams, much larger valves and higher compression ratio. All 6G74 ancillaries fit, no wiring modifications required.
Alternatively, a VQ30DET would be awesome - all-alloy (light!) turbo V6.