The cowl patches are stamped from 19 gauge steel and feature a full patch area from firewall to windshield frame with the vent collar already spot welded into place. The worst rust-through is usually in the valley area surrounding the collar, so these patch panels are usually an effective repair when properly installed. We've seen these installed a couple of different ways; butt-welding is going to be your best method. You can save yourself some time if the upper cowl area at the wiper pivot is solid by cutting around it. This way, you don't have to worry about the wiper pivot mounting being spot on and in the correct place with the patch install.
The patches themselves aren't too hard a hit on the budget at $55 each in the NPD catalog, but labor is going to hurt on this one. Expect the typical Mustang restoration shop to get about 15 or more hours of labor for the repair. You can save some money by removing your front sheetmetal, windshield, heater, etc. in advance, but still expect a good eight to 10 hours of labor with the car delivered in such a state. Depending on your locale's hourly labor rate, you're looking at anywhere from $1,500-$2,000 or more for the installation of these patches.
If your cowl has serious rust-through issues outside of the standard patch panel repair areas, the fix used to be a series of metal patches/strips welded in as needed to repair the issue. Now, thanks to new stampings, the complete lower cowl panel with collars is available for repairing cowl rust issues on '65-'66 and '67-'68 Mustangs. A cowl panel is available for the '69-'70 Mustang as well, but it is complete with the upper panel included. The complete lower cowl panel requires the same amount of tear-down labor for access to the cowl area and just a little more repair labor (figure a couple of extra hours) to remove the complete lower panel and weld in the new panel versus the two separate patches placed at the ends of an original cowl lower panel.
The cowl vent lower panel for '65-'68 Mustangs is designed around the '67-'68 Mustang cowl stamping and will fit the '65-'66 Mustang with minor tweaking. Due to the larger repair panel and stamping of the assembly, the full lower cowl panel is roughly twice the price of both left and right patches, ringing the register for $209. However, any good shop will tell you that you can easily recoup the cost of the full lower panel when you start adding up the cost of labor and materials to hand-make a bunch of small patches and weld them in to repair a perforated cowl panel outside of the two patch panel repair areas. For'69-'70 owners, the complete two-piece cowl is $465.
Last but not least, after you've made your cowl repairs, be it a patch panel or a full lower cowl panel, you might want to consider adding a new upper cowl panel to your parts list. If your '65-'70 Mustang's upper cowl panel is just as rough and you hate the thought of reinstalling a rusty upper panel over that freshly repaired lower cowl panel, then these new reproductions are your answer. For the '65-'66 Mustang, you'll have to buy your upper panel as part of a complete cowl “tank” with the upper and lower together ($434), just like the '69-'70 version mentioned above. For those with '67-'68 Mustangs, you can buy the upper as a separate part in NPD's catalog for $229 if you need it for your cowl repair or restoration project.