I can only speak for the ones on the white car in my first post.
My (old) car, The "ratty" white dart has mini tubs and the springs moved under the frame.
The super stock wheel arches (like mine) DO REQUIRE cutting AND welding.
We used small diameter rod to finish off the raw cut edge and give it a finished look. Just this is a fair amout of work due to it being "hot" work. You cant hammer form cold bar stock to floppy sheet metal so... yeah. That takes effort.
Next up required opening the back of the mini tub and pushing it back about 6 inches. Again a fair amount of work due to having to make the patchs on the english wheel.
We are running a 29x14.5x15 Hooser DOT slick on a custom with 9 inch ford. centered in the factory wheel opening.
So short answer is yes. Lots of fab work involved to do it like this. Its not just cutting but i cant speak for any other size or shape wheel arches.
I dont think their is a true correct way to do it. As far as I know all the SS cars were taken delivery of before the wheel arches were cut.
