69 Dart Custom tire issue

Remember that there is lateral movement as well as up and down.

I would not cut or roll over anything to fit the tire. I would replace wheel / tire. If you have inside room to the spring, then as you pointed out, more back spacing (BS) is the solution. If you can't get the specific BS get one that is too much and shim it outward. But with only 3/4" to the spring, I think narrower tire wheel is your only option