Tires definitly need to be supported full width (+). Here are some simple built up supports I use. The one pictured is for the duals. You can make them any height/width you want and use your scap wood. The plywood top won't crack like a 2x12 and they are much lighter in weight as well.
Rick and Lynda Smith
2004 NEWMAR DSDP M-4011-SPTN 8.9 Liter Cummins
2011 GMC SLT 1500 4WD CREW toad