[Pontiac1976]

In general 81 was the a changeover year for the glass inner trim (upper) some may have older, but most would have the other. The Big change 88- 91 in the Blazer an Burb glass you can see real easy if have then close to each other. If you change over glass and inner trim seals rubber channel seals of the same year. You should have lees trouble in that area. The door 84-91 shells all the same . ** But if some had changed the shell you may have to tweak the opening @ the glass opening ( steel may be a little tight from storage and moving around mostly at the bottom (where the glass first pop out) but sometimes the upper or the side (where the glass makes the seal). Making a wood wedge long enough lavage to pry a little open. (when doing this go slow and in little steps) "The thing is to keep the all same year glass and seals runs your using for your set up". This includes the vent window glass and seals 73-80 or 81-87 then 88-91 you can get a way 81- up to 91. But the 88-91 on 73-80 you need to update your channel to match the 81-91 vents.
Not too often do you ever have to clamp the closed maybe 3% of the time unless some tried break in the door by the door channel to pop the latch or lock.