Why not get wheels with an offset that puts them where you want them? If you change wheel offset so that the outside edge of the tire moves out 1.5" that means you can fit a 1.5" wider tire by using a wheel that extends further inboard. You can more traction, wider tires look better, and the tire is aligned with the edge body better. It solves multiple problems at once.
I run staggered wheels and tires on my '97 GT to get the look I want and get a wider tire in the rear. The only downside is that with directional tires, tire rotations at home are not possible. I can only rotate them side to side on the same axle due otthe staggered setup and with directional tires it would require dismounting the tires and remounting them reversed so I could swap them side to side. Everything is a tradeoff in life.
If you do decide to run spacers, if they are more than a few mm, make sure you get hub centric ones. The wheel center needs to fit snuggly on the hub as that's what locates the wheel positively on the vehicle. The lug nuts just hold the wheel to the car, they do
not ensure it's centered on the hub. The hub usually extends enough to get a few mm spacer on there without any issues, but for anything thicker, the spacer itself needs to fit snugly on the hub and have a lip of it's own for the wheel to center on it.
Here is an example of a hub centric wheel spacer that's 1/2 thick and uses original wheel studs, though you may need longer ones:
https://www.cjponyparts.com/maximum...r-5-lug-mustang-1979-1993-1994-2014/p/WHSP12/
Thicker wheel spacers they have their own wheel studs and get bolted to the original wheel studs:
https://www.americanmuscle.com/mustang-150-inch-5-lug-wheel-spacers-black-set-of-two-yhtwsp0020.html
This is a random pic of a non hub-centric spacer, it just spaces the wheels out and doesn't center on the hub. it's basically like putting a big washer behind your wheels and requires the original hub to extend far enough to still support the wheel. I've used 3mm spacers on another car without issue, but how thick you can safely go with a spacer like this varies based on the depth of the original hub.
To be clear, I'm not endorsing any specific product, just showing examples. Wheel spacers are a bit of a controversial topic and some folks despise them in any form. If you're going to use them, get the right style and get a quality brand. All of the weight of your car is sitting on them.
However you roll with this, just keep in mind you'll be tosing more stuff up on your fenders with the rear wheels further out. I think it's worth the tradeoff to have the tire sit further out closer to the edge of the body, but if you do stuff like drive on gravel roads or long driveways a lot, it may be a consideration for you.
