I've been thinking about you all day. Let's back this up a little. You got 35k on this unit. It's been running fine, until this trip. What were you doing when you noticed the temp going up. A454 should be able to handle a pretty good amount of abuse. So, we're you pulling a long uphill grade, were you bucking a big head wind?
I say this because you don't just blow a gasket tooling down the road. Something triggered this. Did you just fill up with gas? Maybe you got a bad slug of fuel. You say you blew a valve cover gasket. Did you mean a head gasket, or a valve cover gasket. If you blew a valve cover gasket, that implies lots of internal pressure. Going down hill will create a lot vacuum. You have a pcv valve on the valve cover to suck any internal pressure off the engine. If it closes up or sticks you get pressure on the engine. The pcv is routed to the base of the carb, and all of that pressure is routed to the intake.
Anyway. Is it possible you have a master cylinder that is sticking. Thus causing your brakes to drag? If you were going down hill, and were on the brakes, as brakes get hotter they swell and then start to drag. Engine works harder to keep the unit moving. Temps go up. If the brakes cool. It all goes back to normal. If the master cylinder is hanging up. Then the brakes are still engaged.
Did it feel like the motor was laboring, like you were in a big head wind?
Once the unit cools down, the brakes cool down, the master cylinder may act perfectly normal again, until you heat the brakes up. Then, this whole thing starts again. That's why I asked what were you doing when you noticed the temp rising.
Just thinking about this. You got a weird scenario here. DJ