The sheet metal is off the frame so it is hard to tell where the tires will fit in the wheel well.
Its time for a mock up. Put the sheet metal back on and make sure you like the fit and the look.
Given the money and work required to find a new frame and then put a Mustang II on it, it would be worth a little more effort to see if what you have will satisfy you.
But before choosing the Mustang II approach take some time searching here and other places to get a good picture of how happy people are with the various Mustang II and other IFS options.
There are no magic bullets, all the options I am aware of have both advantages and disadvantages.
