[Jeff65]

"I have the four wheel drum brakes and I want to get rid of that old single cylinder."

Okay, you can do that...

"Can I use a dual master cylinder?"

Yes, but it must be designed for drum brakes. Note that most aftermarket kits are for disc-drum.

"Also, What is the difference between the single and the dual besides having an extra reservoir?"

The lines between front and rear are divided, thus if if one mc chamber fails or if a rear wheel cylinder fails, the front brakes will continue to function. There is usually a shuttle valve placed between front and rear systems with a switch that toggles a dash "brake" light to tell you when one of the sides has failed.

With respect to the master cylinder itself, the size of the piston and the valving determine which is the lead section of the master cylinder and which is the follower. Thus, you can't connect front and rear at random, you must hook them up to the correct section of the master cylinder.

"I am looking to get thw wilwood aluminum master cylinder but an unsure if I can use it. If I can get it can anyone help with what size culinder I can use. I've enclosed a link."

I'll have to look back at the link, I don't think so since the Wilwood is likely for disc-disc or disc-drum combinations. I'll come back and edit this reply accordingly. ...delay....ok, I've looked at your link. No, my opinion is that its not the correct one for your application. Think you'd be better off cloning the system for '67 Mustang (lines, shuttle valve and mc).