The early ones are different from later 1928 & 29. The edge where the lower cowl lacing is attached is more recessed on the later ones to allow easy installation of the lacing.
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The firewalls on Fordors and Cabriolets are significantly different than others (have two pieces to the main/lower section below gas tank).
I have no direct experience with the very early 28's, but as far as the differences in the cowl welting location, I was under the impression that the width of the lower cowl panel itself (not the firewall) accounted for the additional width in that area.
Seem to recall that the location of the starter rod hole was the only difference between the early '28 and later 1928-29 firewalls.
FWIW, here's a picture of a cowl assembly I disassembled a couple of years ago. No doubt an all original assembly. I believe it shows the earlier style firewall was assembled with the later style cowl panels that have the wider area for attaching the welt. No doubt originated in a transition period as the left brace on the assembly also had an original block off plate over the hole where an early left hand parking brake would have been mounted. Besides having three rivets to attach the cowl panels to the firewall there was also several spot welds. Seems like mix and match wasn't a problem with Ford.