Every joint in the HVAC system, even blend doors has foam to seal it so you don’t get leaks. When it is wasted you get leaks and it isn’t so strong coming out the dash vents or defrost for example. It is specialized foam for automotive HVAC systems not your home depot seal your door at home...
Honestly, you can get all the parts to fully rebuild it for $650-800 less the blower motor.
It is a big job to pull it all. Why not prepare for that with fresh parts? The foam needs refreshed too, 90% of them it is turning to dust.
We have done a number of HVAC refresh/rebuilds on 80’s, it is...