Trying to make everything fit, need advice

Ok fellas I need some advice and insight here. I've got a 73 dart that's been mini-tubbed, including the outer fender well to allow the 315 tires that I have to fit under the rear end and up into the wells. I'm moving the springs and of course narrowing the rear. My plan is to get the car as low as possible. I've got two issues I need some assistance with:

First: I was planning on going with a skinny wheel (15x4) up front. But part of me is considering going with a 15x7 with a 4" backspace. Will this fit? and with the car low will it have turning clearance?

Second issue: My wheels are still being built, it will be about 60 days minimum before they ship. By that time the weather will be warming here and I'll be wanting to get the finishing touches on so I can get the car back on the road. The issue is I need to get the rear end measurement for narrowing so it can be sent out since it will take some time. My plan was to mount up the wheels and tires, stuff them into the wells and take a measurement from the back of wheel mounting surface to leaf mounts and side to side. With the delay in wheels I'm thinking I need to find a new method of measuring. I'm considering finding the cheapest wheel I can that matches the spec of the wheels I have on order (15x10 with 4.5" BS) and using that as a measurement one side at a time. Would this be possible. As long as the backspacing is correct it should be pretty similar correct?

Any help with this would be great guys. Thanks so much.