shock towers

national parts depot carries a kit as you describe...only thing is, you need to convert to coil overs, as you dont have room for your stock coil springs.i have seen a couple 66 falcons locally with the towers notched for fe 427's, and im pretty sure they still had stock suspension.
If you decide to chop the towers out of your stang, keep me in mind... i have a 65 coupe that had the inner towers notched for solid motor mounts, and i want to section in the missing pieces. (i am also too cheap to shell out 260.00 for the entire repair panel!)

http://www.rrs-online.com.au/shocktowerproducts.php

which I believe is the kit available from NPD.

a lot of options

most of the mustang II suspension types allow you to get rid of the shock towers. look at heidts.com or martzchassis.com. they may be expensive, but i think they are worth it. you can fit a big block or a 351 with ease, plus you don't have to deal with the shock towers sagging.

matt

I don't know where you can find the templates and such, but here is a "how-to" that seems to be accomplishing what you want.

http://www.fordmuscle.com/archives/2001/06/project460/