For you 80 correct?
If you look on Slee's site under the Tech Zone, Installation Instructions, then look for the Slee Rear Bumper (second one listed as the first one comes up with the 60 series and a pic of the raffle rig
) instructions and pull those up, they have the hardware listed.
Now I'm pretty bad with bolts and sizing but looking at the intructions the biggest he has listed are 12mm x 45mm, qty of 8 which sounds right for the frame rail bolts IIRC (4 per side).
You "should" need about the same size 'cuz IIRC the hitch attachment points and the Slee rear bumper attachment points are about the same thickness. Did you get the shims with your hitch? IIRC they space the hitch down the necessary amount to clear the rear bumper. Slee's rear bumper also uses shims.
Here is the link to the install directions...
http://www.sleeoffroad.com/installation/FJ80_Rear_Bumper.pdf