The SM420 trans will work fine with the LS engine. I've got my original SM420 behind a standard Chevy 350, and that is probably an easier swap. Just curious, why do you want the LS engine instead of a regular small block Chevy swap? And why do you want to use the SM420 on it instead of, say, the stock double-overdrive 6-speed manual transmission that mated to LS1 engines in the late 4th gen F-bodies? Although the SM420 is a nice tough transmission, it is extremely gear limited.
