I think its likely the restricted grill openings of the 39. Once the speed is back to 70 it comes right down. 200 under a load is not unusual I don't think. OTOH the rad is a stock mb one from my parts car so it could be on its last legs as far as that goes!

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.