I've been working on this too over the last few weeks too and am goingg along the same lines. However, I've found that you don't need to cut the firewall to get cool high pressure air. There is a metal panel just below the plastic wiper cowl, just cut through this piece & you will have access to the vent holes in the cowl.
*EDIT* Bill, I reread your post & now see you already have the same idea as I