Most of the new flat glass laminated screens are 6mm thick, the original glass was 1/4" ,so the old glass is slightly thicker as it was armour plate type glass
When I did mine I bought the rubber set from mini sport in SA and when assembling I removed the pop rivits on the "lift a dot" bases, fitted the rubber into frame, then I used some mineral turps to lube the rubber. It slipped in easy as ,then held into place with a couple of ratchet straps around glass /frame so I could trim the main rubber to the base frame. The whole windscreen assembly just fell together ,no dramas at all.
You might find that the bottom of the rivits is stopping the rubber from pushing right into the frame. If you have the screw in type "lift a dot" pins then they would difinately stop the rubber from seating