[DanEC]

The heat cross over provides heat to the intake to help with the warm up cycle after a cold start. Most cars in the 60s also had a spring or thermostatic coil heat riser valve in the pass side exhaust to close it down when cold or at light throttle and force exhaust back through the manifold to the driver side exhaust system - which was why the pass side exhaust system back in the 60s would always rust out before the driver side. Once the engine has run awhile the manifold is going to get pretty warm any way from crankcase heat and it's purpose then is mostly nil although if unblocked it can add enough additional heat to promote some carb percolation issues with modern fuels. Some cars had intake mounted thermostatic choke coils that needed the crossover heat to work properly but the Ford 2x4 intake was not one of them. Best to block them off unless you are having cold start issues you can't live with. One benefit of the heat crossover is it does act somewhat like an H-pipe in the exhaust system to help mellow out the exhaust sound - but to a much lesser extent than a true H-pipe. I've never experimented back and forth but I've heard from some of the vintage Corvette guys that they can tell very clearly a difference in exhaust sound on a motor with a blocked crossover versus a motor with an unblocked crossover.