I agree with the guys on the converter, but you said you dropped the timing because of detonation, before or after it became lazy? Was it pinging? With those pistons and 906s it doesn't sound like too much compression unless it's very lean, in both carbs. I always had to do some work on the carb with cams like you have and stock converters. Like one said, if not a dead vert, a tuning issue. Also make sure no wiped cam lobes. Maybe the painter guys screwed up something playing around with the car. All a guess without seeing, touching car.