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.

If you're keeping the small bolt pattern then you should probably use 4.25" backspace in the back, not 4.5". With BBP axles the 4.5" is the way to go, but with SBP it will probably put your tires pretty close to the springs. With 255's you'll have room to the quarter, so the 4.25" backspace would probably work better for you.

The front should be fine with SBP disks.