Ditto to everything Marty posted.

Also, use a non-hardening glass seal/glazing compound (like: 3M Automotive Glazing and Bedding Compound).

I had problems with getting a good fit and seal with a Jim Carter seal and a Chevy Duty seal (on two different 54 trucks). This week, I'll be installing a Steele Rubber seal in one of those 54 trucks.

1948-52 Shop Manual instructions, includes sealer application

Here are Steele Rubber's 47-51 instructions.