Simple rule for Ford Mustang rims onto an E-body
Up to 8.5" wide to figure out your spacer:
if the offset is +44 then you need a 44mm spacer to return to 0 offset. (44mm is 1.73" btw)
When you get to 9" or wider you need to be very careful of offset/backspace because all our cars are built differently and it gets tight on a STOCK suspension with that wide wheels.
Bolt on spacers are the only way to go and they are generally safe. The attaching hardware is critical with them. They need to be the correct length and in good shape.
Also of note is that the center register is smaller on the ford rims. In the rear there's no problem because the spacer will extend past the factory center register of the rear end. In the front, if the spacer doesn't extend past the center register you will need to make a choice, turn the register of the rim or turn the rotor. When I ran 15/16" spacers I turned the rotors. When I went to 0 offset rims again, I sold the unmodified Ford rims (at a profit!) and just bolted my new rims to my rotor, no issues.