Alright, I'm assuming the mixture/timing are in the ballpark. Overheating at speed is a water flow issue unless the grill setup is extremely restrictive blocking airflow which is rare & I dont think thats the case here. I would toss the restrictor for 1 day which will tell you if it is restricting water flow too much. I'm assuming the lower hose ain't collapsing. Sounds like it may need more radiator (not enough capacity or partly restricted). Seeing good flow at idle can be misleading. Holler how it goes. PS good to hear no combustion gasses