Got a tire carrier/30 hub from a buddy here in ND. Tried to think of another option than just bolting it to the bed. I thought about cutting out the crossmember and putting the spare under the flatbed, but the 35 wouldn't fit between the shock mounts. I decided to cut up the second hand mount and weld it to the crossmember and cut the wood around it so that its recessed into it.
Just have to cut the wood out around the tire so that it will drop in and mount to the carrier. Then figure out how to strengthen the wood after cutting it.
I would run some cross boards to tie-in the shorter cut boards with the longer boards that have more mounting points, unless you can run a thinner board underneath the tire and above the crossmember.