I know the 6.0 with a 4L80E has a spacer between the flex plate and crank. I'm pretty sure using that spacer and flex plate will work. The bolt holes for the torque converter may be slightly off but some guys have oblonged those holes when putting 4L80Es in place of the 4L60Es in the newer GM trucks.
advance adapters also makes a kit with a re-drilled ls flywheel for the th400 tourque converter and a spacer for the crank.
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Project EARTHQUAKE:
72 Nova SS, Plans have changed a little:artmorrison front subframe, and rear 3 link, pretty much stock ls1 and six speed with a single turbo for around 550hp. with a few other things mixed in.
current cruiser: "The green machine"
'70 Impala 4drht, 26K original miles, 2" drop springs and large swaybars, drives pretty good for a land yahct
and why do we modify everything...."BECAUSE STOCK SUCKS"
So it looks like using the ls/thm400 combo the weight comes in around 551lbs. and you keep your stock speedo drive and driveshaft length. Plus ls motors can be run to to 6500 rpm w/no drama. Aside from the tci flexplate and maybe a crank spacer, do the LS adapter engine mounts allow for a close to stock engine mounting location? It also looks like a rear sump oil pan may be needed, but other than that. Not too bad of a swap now that there is a distributor kit available for the LS engine and carb style manifolds are available.