Back to your original question - will an ML7 cross slide fit? Are there any Myford owners near you? Replacement with an ML7 cross slide would be easier and a much more satisfactory long term solution.
If you've got it in bits you can take the bits to a Myford owner - no need to borrow bits.
I'm not a Myford owner - can anyone else help?
Russell
Actually there are ( to my knowledge)- TWO ML7 cross-slides. One is the standard short one and the other is longer.
However, there are (at least) two more. One is on the Super7 and the other on the '10'
To confuse further there are TWO more. One is the fixed vertical slide one and the other one goes on the remaining pivotting slide

You clearer now?
Now this cracked ML7 tee slot thing is nothing new. Martin Cleeve- author of Screwcutting in the Lathe cracked his and if you look carefully at what is on the front cover his is NONE of the foregoing, it is a Myford Special and isn't cast iron. It is steel. However, he extended a slide and he mended his cracked one and he fabricated another.
I've lost count.-forgive me.
All that I would suggest is that you measure YOUR broken slide- width and let us know the length involved. I have a long ML7 top, A Super 7 one and a fixed 'funny' because it probably came from a Zyto but seems to have all the markings of one from a ML7.
PM me, if this makes a slight amount of help.

Norman