flex plate

The 4l80e is similar to gen-1 style automatics and thus sits further away from the motor. GM created a spacer in order to adapt the 6.0L and other ls based motors to this trans. The spacer works to connect ls style motors to old style transmissions as well (TH350, TH400, 200r4). However since you are swapping for a newer style transmission I believe you need an ls1 style flat flexplate with no spacer. TCI PN 399753 or use stock gm flexplate for ls/4l60e not the ls/4l80e flexplate your motor may have come with.

More info can be found here

http://www.ls1tech.com/forums/autom...-style-trans-th350-400-2004r-700r4-4l60e.html

http://www.carcraft.com/techarticles/ccrp_0810_engine_transmission_swap_gen_3/torque_specs.html