I think that you should do some test joints before jumping into the actual frame. Get some cheap lugs and tubing. Take that joint as practice and cut it up to see what sort of penetration you got. Maybe the gaps are only on the shoreline, or maybe the filler didn't even go very far. You can get some firebricks to help contain the heat, but as was said the heat from that torch is minimal. Even the Talbot book says it is unlikely to work well for the heavier BB and crown joints.