Match Drilling
I’m assuming you want to locate holes in the Cowl for placement of the fasteners, not vice versa. The technique I propose works either way with a little adjustment.
Good chance for getting an acceptable result “over the arch” if you pay attention to what the material is doing while you work through the process of matching the dimples.
Drilling through the dimples, before or after "undimpling," is a bad idea as it will expand the holes.
I'd consider using a “transfer strip” to achieve your matched hole goal.
There is an article in SA archives by John Thorp in his "Building the T-18" series (Article #2 or #3 back in the ‘60s) that describes the technique of “Matched Hole Tooling.” I’ve taught this technique at forums given at SnF and OSH.
Cleecos, by themselves, for this task, are inadequate for achieving close tolerances. If working “in the flat” John recommended making “holes at the corner 1/8" (0.1250") and using an AD-4 rivet as a locator.
The fact that the holes are already dimpled denies you that advantage but I include its reference for understanding my point and for ease and convenience in other circumstances.
A finnicky builder, if very careful, may avoid the shifting of the transfer strip by handling very carefully and using lots of clamps along and across the transfer strip.
Matching the holes is problematical because accurate transfer of the position is difficult using the “standard 0.125" transfer punch. Consider (machining preferable to hand) creating two or more "custom transfer punches" reducing the diameter of the shaft of an old drill to ( say 0.145" or so) and carefully adding (if by hand) a center point.
Once you transfer punch all the holes, go back, viewing perpendicular to the holes around the “arch”, verify the strip hasn’t shifted and the punches are concentric to the holes.
You then punch/drill the transfer strip with #30 or #40 drill. If you went light on the forces while using the transfer punch to locate hole center, the shallow dimple can be increased to better locate the drill for the next step - transferring the hole location to the Cowl. If this is the case, you could consider using a Whitney Punch. Let the “nib” of the punch center into the transfer punch depression and very carefully, increase the dimple to the depth of the point on the punch.
I’d also consider initially drilling (in my case) the cowl with #40 and take it over to the firewall, carefully position and check concentricity across the arch. If something was “off” you can adjust by angling the drill when you go to “full” size.
Hope this was both "on point" (just read the reference to hinges) and clear!