Parts store I think

I need to run a test this morning and re-verify that it won't go down to the t-stat temp at idle with both fans running continuously.
(Much less heat to reject at idle vs. the 20-30 hp required to maintain 60 mph).

If that is the case, I almost certainly need more radiator.

Highway overheating could be insufficient t-stat flow (as I think you're implying?)