Sounds a lot like a leaking fuel injector to me. I had exactly the same experiance happen with the L28+M90, blew up the sheetmetal plenum like a balloon. Combination of enough fuel in the intake plus slightly too advanced timing/valve not sealed/some source of ignition in the intake. I spent two months hunting down that problem, the injector didn't leak while sitting still, only when the car had been running and was hot-once cooled off, the drip stopped.