My plan is to finish/paint the inside of the cowl and then glue the panels down. I will spray a washable sealer over the finished cowl that will wash out later. This way I can spray primer and paint that will not stick to the inside of the cowl. The glue I will use has little beads that will set a thickness of glue to hold the panels in place. So if I do replace the cowl prior to blasting, I will have to cover up all the openings to prevent water and sand from entering.You can keep sand out of the cowl easily enough with duct tape over all the openings(hand-sand the vent fins at the windshield)....and even if it gets in there accidentally, you can just vaccuum it out by snaking a hose into the vents from below...you just want to avoid blasting through the fins at the windshield. As for the modifications required for the 67 lower...yes...they are rather minor, but I had trouble with the windshield wiper holes and had to enlarge them and cut a small keyway with a metal file for them to work with the 66 wipers...trimming the front edge is no big deal(just don't cut too much like I did...luckily not a big deal since I have that rectangular tubing running under there)
Thanks for the tips on what to look out for when replacing the cowl panels. Those are the things that can really make the job easier and less painful. I will try to take pictures of all the little things needed to make it fit for others that may need the information.