manofmerc, do you recall what the bore sizes of your Mustang 11 master cylinder was, and what the Wilwood master is? And is your Wilwood master cylinder the plastic and aluminum MoPar style, or their own billet unit? I am currently using a manual brake master cylinder on my 59, that was originally for a Maverick, it works, but does take quite a bit of pedal effort. I seem to recall the bore on the Maverick master is 15/16".