Many sanctioning groups- like NHRA- require burst panels on blower manifolds. They are designed to rupture at some value greater than normal boost pressure, to avoid (or at least reduce) collateral damage. When a burst panel is not required it's common to just provide a small spring-loaded popoff panel on the manifold, which will handle minor backfires. My incident was so violent that if the poorly-welded panel hadn't sacrificed itself, the blower would probably have "lifted" and hit the ceiling, even though I did have a popoff valve on the rear. I had to scramble to extinguish a dozen fuel/oil fires all 'round the garage.
The guy who does blown-alky tuning at the dyno shop is sure that my setup was way too lean. Apparently the engine software I use is off-base for extreme situations like my monster blower on a tiny engine.
Hopefully, I'll wind up learning a little something.