After you fellow restorers have finished your trunk and floor pan installation, please advise on what was used as seam sealer and how it was applied and spread?. Anyone use 3M-5200?

Don't remember the part number off hand, but I'd use 3M's Brushable seam sealer, and apply it with an "acid brush"

make sure you primer the metal with a good quality etching primer or epoxy before applying the seam sealer. One of the best seamsealers available IMO is a urethane manufactured by 3M called Ultrapro. I avoid the brushable can type and toothpaste tube style, old technology with major shrinkage problems. The absolute best type is the two component style available from SEM, 3M, Kent, and Fusor. But It will run ya about $35 for the kit and requires a special gun for dispensing the product.

Yea, I like the ultra pro too, but I wouldn't be as concerned with trunk, and floor seams. Ultra pro is all I ever use on exposed, painted seams, such as rocker to quarter seams, etc.