interstate driving = overheating?

[zz3guy]

I have a gearhead friend with a problem we can't seem to solve. It's overheating when driving down the interstate at 70mph even on cool days.

It's a 40 Ford street rod. He has a Pontiac 400 engine, Quadrajet carb, 350 turbo, "highway" 3.08 gears, aluminum Griffin radiator. This car has been dead nuts reliable since getting it on the road 2 years ago & it doesn't overheat in traffic & runs cool (190-195) on the highway at 55mph. But hit 70 on the interstate & she keeps getting hotter until he has to either pull over & shut her off or hit a highway to keep the speeds down. Any ideas? I heard timing would have an effect on it but I'm sure from driving it that the timing can't be off by much because it runs great & never pings or detonates. It also starts & shuts off just fine too. My pal is at his wit's end on this one & so am I. Anyone on here had a similar problem?