Which LS motor mount setup do people use in a 67-72 C10?

Two common routes:

1. LS adapter plates + standard small-block mounts. Adapter plates bolt to the LS block and take a classic small-block Chevy 3-bolt mount; different hole positions move the engine fore and aft. Flat adapters like ICT's give several mount positions and raise the crank centerline slightly (0.177 in in ICT's case — note it if hood clearance is tight). This still works well, especially if you're moving the engine from the stock location.
2. C10-specific LS mounts. Speedway designed C10 mounts that skip the adapter and use stock '98–'02 F-body motor mounts, attaching to the frame brackets with one long bolt like standard GM mounts.

For the transmission, an adjustable swap crossmember that uses the factory frame holes (with slotted plates) handles almost any transmission. See the C10 swap guide.