1968 Airbox rebuild question

This is really helpful and I thank you for taking so much time to help. Yes, first time. Bought the car in 1984 while living in California. My plan was a slant six to 360 engine change. One thing led to another...
No cigar. The metal plate that attaches to the box where the heater core tubes come from the inside, does not stay put, or screw in flat, when the tubes are pushed through. Instead, pushing the tubes through breaks out the piece between the two tube holes...where the screw goes. I re-epoxied it but am at a loss as to what I can do. Without that plate, the tubes fit.