But, thinking about all this, can 10" rims fit for the full extent of travel? Are you talking 15" or 17" rims or...? The cross section shape of the tyre will be the issue.
I thought that 69/70s could manage 9" rims quite well, I believe some people have gone to 9 1/2".
Regardless of this issue of who ends up paying for what, the point is that with the Mustangs being relatively light cars at back then super wide tyres no matter how hunky they look - & I'm sure yours will! - just end up causing traction problems so surely the exercise starts to become counter-productive unless one drives around with a few bags of cement in the boot.