1976 K-10 runs hot down the highway

I have a 1976 K-10 pickup. Just installed a 350, .030 over, appx 9 to 1 compression, around 300hp that a friend (and I) built and put in his truck, put on about 500 miles, then put in my truck. All is fine when running around town. Runs at 185 deg and stays there well. Yesterday I went down the freeway for about 20 miles and it slowly and gradually got hotter and hotter (up to 220 deg) and started to run ****ty till I got off the freeway. Then it went back down within a few miles to 185 deg. and was OK. Same thing on the way back home. I know these are good: thermostat, gauge (mech), cap, radiator is clear, new 50/50 mix coolant. I have a stock 2 row radiator and with the other engine I had (about 200hp, 8 to 1 comp 350) it ran hotter as well (up to about 195-200) on the freeway. All the guys I know with 4 rows have no problem. Should I try one with 4 rows or could this be something else?

It sounds like you have too many horses for that stock radiator.  Go with the 4-row.