Tire/Wheel Experts Help Needed

Hi Guys,
I've been reading a lot of threads and comparing tire size charts. I am looking to buy new rims and tires for my 69 Barracuda. I do not want to change or alter the suspension. It has stock 8 3/4 SBP rear, stock drums up front, but I do have a set of stock discs/spindles SBP on the shelf that I will likely install when I get the tires and rims. Not sure if drums or discs would alter my backspacing for the fronts.

Here is what I have come up with and believe will work but your expert input is requested;
Rear 255/50/16 on 16x8 rims with a 4.5" backspace
Front 225/55/16 on 16x7 rims with 4" backspace

I'm sure the tires will fit providing I got the backspacing correct, IF I understand this backspacing stuff correctly.

Your help is appreciated.

P.S. For those that are going to ask where am I am getting my rims for a SBP. American Racing will still cut a 5x4 bolt pattern. They just don't advertise it and it is a special order.