convert single to dual reservoir

Mine is out of a '70 Mustang.

HistoricMustang
www.historicmustang.com

74 maverick part works also. Cheap and easy to find at parts stores.

dual reservoir

It's a good idea, but a few details need to be addressed. Is your coupe power assisted. If so, the details are complicated since the original booster and a dual MC won't fit between the firewall and the shock tower. There are conversions that use more compact boosters, but they require considerable FAB.
If the coupe is manual brakes the job needs a proportioning valve and adaption of your existing hardlines. As posted previously a manual MC from a Maverick/Granada bolts right on and has the same push rod configuration as the original fruit jar MC. They are available new for about $45. Don't use a rebuilt one.

See member degins auctions on EBAY to see a full disc brake swap kit for your 66 which include everything from a new master cylinder to dust caps and everything in between except the hardlines for about $600.

You also need to put the correct one in depending on if you are drum/drum or disc/drum as the M/C's are different.

A dual reservoir MC in concert with a distribution/proportioning valve is safer than a "fruit jar" MC because the front hydraulic circuit is isolated from the rear. This is not a result of the dual reservoir, but of the distribution part of the distribution/proportioning valve. A leak between this valve and one of the wheels will result in a pressure drop in the valve which will cause it to close off that circuit (front or rear). The proportioning part of the valve just introduces a pressure bias (lower pressure to the rear drums) front to rear.
Original drum-drum distribution/proportioning may be available for 67 and up Mustang, but disc-drum ones are much more common. Most of the aftermarket disc brake swap systems use a metering valve as a proportioning valve (Summit type, these do not stop leaks).
I would say that swapping to a dual reservoir would only make sense if you could find an original drum-drum valve from a 67+.

Most Mavericks have drum/drum brakes. The hardware bolts in without mods. Some bending will be required for the brake lines to re-attach. I would swap out the fruit jar master cylinder for safety sake if nothing else. Total cost of swap is cheap, and can make all the difference in the world when a wheel cylinder or brake line gives up at highway speeds. While you are searching for parts, a disk brake swap would really make a difference and is a simple bolt in.