Can a sill plate be larger than 2" thick? I installed a footing to carry a point load. I will be installing 3 LVL's on top of this and then trimmers to support a header. So I need the need to get my footing up to the bottom of the existing joists. I am a couple of inches short. Can I use 4 X 8, or even rip down 6X to get to the height I need? Would I be better off using 2X and furring up with PT plywood?
Yes, sill plates can be thicker than standard 2x material.
Sill plates are used to transfer the wood framing to the concrete foundation for 1) shear, 2) gravity loading, 3) seismic loading,
That transfer of load is usually fastened with bolts (sometimes metal clips poured into the concrete stem wall). Just make sure the bolts (and washers) are large enough that they don’t bend.
In your case, the solid member is better than the multiple built-up members, but either would work, just make sure the vertical post connection is made to the member connected to the concrete...not just the built-up members.
Btw, only the member touching the concrete needs to be treated.