5.5" backspacing isn't optimal for an 18x9 wheel on a 68-72 A-body. 5" would be much better for both ends. My suggestion for tire sizing would be a 255-275 on the front and a 275-295 tire on the rear.
Post some photos when you get them on the car.

 

What about the +14mm offset? Is that a factor?

 

Not really. Zero offset on a 9" wide rim would be around 5" so 14mm is close to an extra 1/2" = 5.5". As I said before,an 18x9 wheel with that backspace would probably fit well on the front of your 71 and would be ok on the rear.

 

Can someone please explain what how "offset" affects backspacing? Wheels come in a choice for both backspacing and offset. And in prior post backspacing is referred as "offset", I believe they are not one in the same.

 

I saved a thread about this from Dale McIntosh (TC #92) about the differences between offset and backspacing. Hope this will help.
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I've been trying to do some extensive research on wheels myself - particularly on the American Racing site since they seem to be the most comprehensive. Take the custom offset 17" TT-II for 4.75" bolt pattern as an example. They offer no less than 20 in 17x11 with the following offset
(1st number in millimeters) and backside setting (2nd number).
To quote ARE's terminology:"Backside setting/Back spacing - The measurement from the mounting pad to the inner edge of the wheel"
"Offset - The offset of the wheel is the distance from the mounting pad to the centerline of the wheel."
"Negative offset - When the mounting surface is closer to the disk/drum (Inside) of the wheel. Or the mounting pad is behind the centerline of the
wheel. This type of wheel is commonly referred to as a deep-dish wheel. This offset is common in older vehicles and many of today's trucks and will bring the wheel/tire combination out away from the vehicle."
"Positive offset - The mounting pad is forward of the centerline of the wheel, towards the street side of the wheel. This is common in most front
wheel drive cars and some newer trucks. Generally speaking a positive offset wheel brings the wheels closer to the center of the vehicle."
Point here being that there is more to consider besides just backspacing:i.e.
How far the mounting hub is offset from the center of the wheel makes a difference,as well.