The pedal shafts on my '37 are the same as yours with that groove. Since the grommet is on the engine side of the firewall, it doesn't need to slide on the pedal shaft. If there is a gap after installation (there may not be as the grommet stretches over the shaft) perhaps one could fill in with a bit of black silicone sealer.

I didn't know about these grommets when assembling my truck so I don't have them. I will probably wait to have another reason to pull the floorboard before considering adding them.
Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix