Ford uses a funny little hollow bolt through a type of elbow fitting to match up to the hard line that comes off the booster.
A junkyard is undoubtedly your best bet. Doesn't have to be a Mustang, since many Ford and GM cars of early 90's and up vintage used a similar booster.
A regular plumbing shop could probably figure out a simpler way to do this, but here is another alternative:
Can you figure out the thread size/pitch for the fitting?
It is not pipe thread, and if you were forced to come up with something on your own you would have to find some way to prevent it from leaking. It is a low pressure line though and so I don't think it requires anything really fancy. Once you determined the correct thread size/pitch, you could use a flat surface sealing ORB fitting similar to this:
The o-ring-boss end would seal against the face of the booster where the fitting attaches. The other end of the elbow is (female) NPT which would allow you to screw in whatever size hose barb you require.
The only thing you would need to figure out is the thread diameter/pitch for the return line fitting. I think it is around 5/16" and some variety of SAE thread.
Bart, do you have the original fitting on the hose?
After getting frustrated trying to find something to work there, I cut the fitting off of the donor hose leaving about 1.5" of steel tubing. Then I did the first step of a double flare to get a bulge. Now I have 3/8" transmission cooler hose secured by a hose clamp.
I have not run it or put in fluid yet, but since it is the low pressure return, I think it will be OK.
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