340 Engine issues

Hi guys, I just recently rebuild my engine. I had an over heating issue that I think I resolved with a new distributor with vacuum advance. When I take the car out for a drive, she seems to be running great but as soon as I go over 30 miles the engine shakes valiantly. I was thinking timing but if I adjust the timing any lower the heating issue returns. Right now its set to 12 initial, and advances to 28 with vacuum. Balancer was replaced last year with a summit OEM replacement (PB1004N). I used it on the engine last year before the rebuild and was fine. Only difference now is the aluminum heads. I should also mention that my compression is pretty high for daily driver, 11.1-1. I have 87 octane in the car right now but will be pulling it out today and topping up with 91 Octane. Any suggestions???