Is the whole setup original style? If anything has changed to reduce pulley wrap that has an impact. The fan seems to be toughest to drive.

If the fan clutch is extra tight or locked up that could be the problem. If your are running a viscous fan clutch (hope so) it should slip to a certain extent at higher RPM to help prevent fan overspeed and belt overload.

Also, FWIW, I believe I have run 3/8" belts in pulley's designed for 5/16" belt w/o problem.