18" Wheel help
Nice '71 Demon! You probably don't want to cut up the body or mess with it to get 18" wheels on it. The only reason why there are a lot of Asian aftermarket wheels with a 5 x 4-1/2"[ 114.3mm ] Bolt Circle is because it's what Mazda, Lexus, Acura, Infinity, Nissan, Toyota, Honda use on various car lines still use. Thank everyone copying 1940's Ford designs. Most will have too much backspace, but if you search for wheels that fit Nissan 350Z you can get an idea of what's out there. And they kind of set what + offsets you will be able to find, because the benchmark wasn't set for old Mopars.
You sometimes will have to make a decision if the Hub Centric's will clear each other, or if you want to use Hub Centric Rings to span the gap or not. Or if you have to machine the wheel's Hub Centric and then make your own dust caps or find some off the shelf that will work.
If you were building the car from scratch, I would change it to coil over suspension on all 4 corners and ditch the leaf springs and torsion bars. The advantages of tuning your ride height and tire clearance in the back wheel wells is too good to not do it.
Your car is too nice to hack. So mini tubs and 355 width tires would cost too much and not be worth chasing a look. And then those fancy 18" rims will show stock tiny Mopar disc brakes or your drums in the back...
You can have wheels widened too to fit the back as you go along, say you mini tubbed and want to keep the same wheel to increase your backspacing.
Places like...
Weldcraft Wheels (Michigan)
Or
Eric Vaughn (California)