best way to tell if it's vapor locking is to poor some cold water over the carb,pump and line and see if it will start.It's basically boiling the gas to a vapor in the line.
Our old international farm truck dumper would run fine for about 30-45 minutes before it started vapor locking.The old girl had a cracked manifold and no exhaust.Simple fix was to re-route the fuel line away from the manifold and run new high pressure rubber fuel line.Your fuel line runs under the manifold and down to the carb.If you re-route it down the other side of the water pump and down around the crank pulley and up to the pump it won't vapor lock any more.
Thanks for the info. The line has been patched on both sides of the fuel pump already. I'll look into rerouting it when I get home.