If you have the stock, unmodified
suspension, then the swap MAY work with no problems. If you have lowered the A-arms and have lowering springs, you may encounter excessive bumpsteer or accelerated tire wear from the installation of the '74-'77 Maverick front disc brake assemblies.
This brake setup is the very same setup that was used on the '75-'80 Granadas and Monarchs.
This disc setup isn't a problem when installed on a '67-'70 Mustang because they have the very same track width and
suspension geometry angles as the Granadas. The '65/'66 Mustangs have a two inch narrower track width than the '67-'70 Mustangs/'75-'80 Granadas and the steering arm angles of the Granada (Maverick) spindles are slightly different than the stock '65/'66 Mustang spindles.
I've installed these front disc setups (from wrecking yard donors) on a '65 coupe, (2) '66 fastbacks, a '67 coupe, a '68 coupe, and a '69 fastback. The only one that I had a problem with was one of the '66 fastbacks that I had also lowered at the time I did the front disc conversion.
There's a company called CSRP that makes brand new reproduction (castings) of the "Granada" front disc brake assemblies. They have also recently made a revised casting of the Granada-style spindle to specifically work with this setup on a '65/'66 Mustang chassis. With the revised spindles, you would have no more bumpsteer problems with it than you would have with your stock '65/'66 Mustang spindles.
CSRP's spindle listing:
"1964-66 spec Granada type high strength alloy steel steering knuckles (spindles) , includes grade 8 mounting bolts. has 1964-66 correct steering and
suspension geometry. Sold as a left and right set only.
P/N GKK.65 $179/set"
http://discbrakeswap.com/Mustang Disc Brake Conversion Kits HOME index.htm.html