Looking at D150 terrible spark knock

I am looking for a D100 or D150 long bed truck as a personal work truck. I looked at a 1984 D150 tonight that has no rust (rare here) 318 motor was rebuilt 10,000 miles ago and 15 years ago. He said flat tops and built by a shop with receipts to be 9:1. He also said the heads were decked and valves were enlarged to 2.02's. When I drove it and gave it about 1/2 throttle not letting it kick down it had very bad detonation. It was not hot and it is cool out here tonight. He said Oh Yah it has always done that I forgot to mention that I just don't give it that amount of throttle I either back of or let it kick down. I decided to pull the vacuum advance just to see if it made a difference. It didn't. I am thinking it is running lean.He added an Edelbrock 600 cfm carb at the time of the rebuild. Truck doesn't smoke and had good oil pressure so I am leaning to take a chance on it. Looking for opinions.