See wad'I tell ya, FSM (1977) describes exactly how to do it, sounds like someone F'it up from the beginning.
FTW couple of things I left out before, popsicle stick and the black windshield sealer tube.
Correct me if I'm wrong, after removing old glass and prepping 'pinch weld' think that was what the primer was called, pinch weld primer, check, yep but 3M calls it something else now.
It's use, (wear appropriate protection kiddies)
And if you don't do it right, BOOOO!
After the cleaning & prep position glass in pinch weld before installing butyl tape/strip and position properly using any rubber bumper blocks/spacers (you did save those right?) if they were there. Use 4 pieces of masking tape, 2 top 2 bottom to keep the proper alignment so when you lay the glass on the buytl strip as because you ain't gonna be able to shift it around once it hits the strip. Slice masking tape and remove glass to a safe spot, lay the butyl strip down with the seam IIRC at middle bottom of glass, check FSM they tell you were the seam goes, bring your glass back and lay it very carefully on the butyl strip aligning to the masking tapes, press lightly around all edges to firmly seat glass to strip (look for voids). Then load up your caulk gun with the proper sealant for the glass to butyl strip, lay a fairly healthy bead all along the glass edge to butyl strip, then take the popsicle stick and smear/spread all along the glass edge to get it into any voids, (there really shouldn't be any, but on a ancient car who knows what damage has happened to the pinchweld)
*NOW* The important step, as per the FSM or sealant instructions, spray a light mist of garden hose water all along the sealant seam and look for leaks. If you find any take your popsicle stick and smear the sealant around till you have no leaks. IIRC part of the water spay misting with the right windshield sealer is that the light misting of water also helps speed up the cure of the sealant.
Now don't go heavy handed with the sealer in the more must be better attitude, you do want to get the trim back on (and off) easily don'cha?
Take your time & Good luck!