'69 Barracuda T56 swap, floor, cross member
Used 3M Panel Bond to seal the perimeter of the floor. The stuff is sort of watery and seeps in between the panel joints. In several places had to add some more because it seeped down in there.
Note this is a factory AC car the floor heat vents are very low on the transmission tunnel. But I was able to get the panel bond up in that seam.
It took two tubes of panel bond, not cheap, but the stuff dries to a super hard durable plastic.