Can DOT 5 be used in a Mustang II

dot 5.1 is ok,but dot 5 should only be used in systems designed for dot 5

DOT 5 has some advantages over DOT 3, 4, and 5.1:
-It doesn't absorb water as easily, making it more stable over time.
-It doesn't eat paint if you spill it.
-It has higher boiling points so its better for racing.

DOT 5 also has some disadvantages:
-It is more compressible than the others possibly giving a softer pedal.
-It is not compatible with other types of brake fluid.

A good article on replacing DOT 3, etc. with DOT 5:
DOT3/DOT4 to DOT5 Brake Fluid Conversion

A couple of good articles on why or why not to use DOT 5:
DOT 5 - Wikipedia, the free encyclopedia
StopTech : Balanced Brake Upgrades

Personally I would not use DOT 5, but would use the factory DOT 3 and flush every couple of years by opening the bleeder at each brake caliper and cylinder while refilling from the top.

In your case I would either flush the system with more DOT 5 (opening each brake caliper and cylinder bleeder in turn), until I was sure I had pure DOT 5 in the system, or I would drain it all out and refill with DOT 3 and do another flush in a few weeks. I don't think it matters much at this point which one that you do, so choose the easiest for you.

Its likely that as long as you have all of one type fluid in the system you are going to be fine. Here is a thread on an antque car forum that bears that out: DOT 5 Brake Fluid??? - AACA Forums

Personally I wouldn't be surprised if you needed to change out a master cylinder in a couple of weeks, but at least that's a relatively cheap and easy fix.