I've done this job on a 70 Coronet.
i.e replaced the entire trunk floor,
outer wheel houses, and trunk extensions
then seam sealed it all..

Here's what I did and it's been on there for 8 years now, no issues.

This is my 2 cents only.
Others on here are experts they may tell you differently...

FWIW,

The 3m Brushable stuff shrinks a lot!
IMHO, You'll be swiping that on for days..Skip it

First, get a self leveling seam sealer(usually in tubes or 2 part
tubes that use a special gun...SEM, LORD FUSOR, EVERCOAT make this stuff as does 3M.) My personal choice is either one of those 3 (not 3M). The evercoat is the most reasonably priced I believe.

The reason for using this stuff first is, the self leveling
will flow as deep as it can inside the seam and really seal it up..

Afterwords go over it with your brushable seam sealer.
I used Wurth brushable seam sealer...
you can get it at www.PelicanParts.com (its a Porsche/BMW site)

Plan on doing two coats over every seam..

For the first coat, I put it on with one of those body filler spreaders cut in half (about 2 inches wide)...
Waited a couple of days then put on a second coat with another
spreader cut about 3 inches wide,

immediately after the 2nd coat I went over it
a parts washer brush, but you can also use the acid brushes too....

The guys that put that stuff orignally must have
been union guys paid by the quart!

There was so much of it originally, that duplicating the
thick stuff they used back in the day in 1 coat is impossible ...

But, that is just my opinion...

You can use the 3m brushable if you want...
A lot of people swear by it... (I swear at it!
Transtar also makes it and its cheaper than 3M
Whichever brand you do use (most especially the 3M)

WEAR A PAINT RESPIRATOR (THE ONES WITH the charcol filters on it)
If there is any product made by 3M that should have a plain old
skull and crossbones label on it-
it's that canned brushable seam sealer! (I kid you not!)

Anyway this is just my

Best of luck to you...