I'd used seam sealer on the top lip of the firewall, seems to be working fine so far. The only worry I had was it bridging over to the dash side of the firewall and causing problems when I spot welded. I kept the bead towards the engine side of the firewall lip, worked like a champ.
This is what I used, pretty easy to work with, but gets everywhere if you aren't careful. Worked well on sealing up the firewall overlaps in the engine bay and smoothed out some.
If you look in the mirror you can see the bead that formed when I clamped the pieces together for welding (seam sealer is light gray):
This was the start of my hotrod education and how people don't do their jobs. The dash was ate up, they covered it up with body filler and pieces of scrap metal. This is also when I realized I had to pull the front end apart to pull the a-pillar covers. It was an expensive education..
Here's a close up of the foam, I had no idea what it was, so went with seam sealer. The foam was on the passenger side of the firewall, I went on the engine side.. same difference? Also note, there's divots, or wells in the firewall lip, those are so you can install windshield trim clips and the screws don't penetrate the firewall. You need to somehow mark their location so that when you go to screw or rivet trim clips back into place you know where they go. Another lesson I learned too late..
If you're doing the work yourself, take notes:
-Mark where the trim clips and tack welds are so you can put it back like stock.
-If you are doing the work, shield your vent windows/crank windows with cardboard, welding/grinding has left their mark on mine. A blind person can read my vent window now.
-don't throw anything away until you're done!! I'm still sorting through buckets finding little screws that I thought were insignificant.
-get a front end fastener kit from Speedway Motors. You'll be cutting the heads off more bolts than using a wrench for on the front fenders/wheelwells, this will take a little bit of the stress out of the operation.