Someone tell me I ordered the right thing please

You might as well get used to the idea of moving the springs AND narrowing the rear end, lol.
Try this;
Remove a wheel, support the axle at ride height. Measure from the inner edge of the outter lip to the back wall ..... where the sidewalls will be. You will probably have to drop plumblines to a sheet of cardboard taped to the floor.
Subtract 1 inch for tire clearance (total for two sides), then convert your measurement to metric by multiplying by 2.54...That will be the maximim size of INSTALLED sidewall that you can fit in there.
Now, realize that when you buy a tire these days, the number stamped on it, was arrived at, by installing it on a rim that was 70% of the stamped size..
For example;
a tire designed to be a 255, was installed on a rim of 255 x 70%=178.5mm, = 7'' in USA measurement. Then the tire was verified to be in the range of 250 to 260 mm wide, and stamped 255.
If you put that 255 tire on an 8" rim, the section width will grow about 3/4 inch, which is .75 x 254=19mm, so your new section width will be between 269mm and 279! So don't screw up!
IMO for long tread life, the 70% rule is too small and should actually be closer to 80/85%, the bigger the tire the closer to 85%.
So let say your measurement is 12inches which, less the 1 for clearance, is 11 which is 279mm.
Now lets say you want to put that 255 on a rim of 80%. That would be about 223 mm or 8.8. The closest size to that is an 8.5(216mm).
Now to that, you add your anticipated sidewall bulge of about 1.5 inches total=38mm. So now yur up to 216 plus 38=254mm there you go. The closest size to that is a 255 which has an allowable range of 250 to 260 on a 7" rim/ perhaps 269 to 279 on an 8" rim.
Will the 255 fit?
@toolmanmike says yes, but just barely. And I'd be willing to bet that he was very nervous when he ordered his bling.
That's one way to do it.
Now, to be accurate;
The distance from the floor to the widest part of your installed tire; when you put that into the wheel tubs, you will find (probably) that where it hits on the back wall, is not where it hits on the outer lip. This will depend on your ride-height. Take a close look at Toolman's Dart; the rear of the car is a lil higher than stock, and so the interference point, on the outside, is up past the widest part of the tire, where the sidewall is curving back towards the tread. This is sortof a normal situation. But if your car is lowered, you HAVE to keep this in mind and you may run into clearance issues that require rolling the lip to cure.
Furthermore; just because the math says it will fit into the tub, it will only happen if the backspace allows it to, and usually, the springs will have to be moved over.
If you have NO DESIRE to move the springs, then you have to subtract the interference factor from the measurement to the backwall.
Earlier I used 12inches in the example, from the lip to the backwall, and allowed a total of 1" clearance. So if your springs are .5 inch to the outboard of the inner wall, you gotta subtract that from the 12inches, and get 11.5.
However, that .5" automatically gives you more than enough inboard clearance; so where I used 1" total, you can now use as little as 1/8 to the spring. So on the inboardside your total deduction is .625. On the outboard side, with your rideheight like Toolmans, you can reduce your clearance to 3/8. Therefore your total deduction is still 1inch........ but your backspace will need to be recalculated.
Now, if your car is lowered, the outter lip will drop down towards the widest part of the profile, and you may need to allow an additional .5 or more inch for that.
And, if you like to blast around the turns, the body is gonna roll over to the outside of the turn , pulling the inboard side of the body, towards the sidewall. So you MAY have to allow for that as well. I solved my problem (my car also is lowered), with poly bushings an extra mainleaf, plus clamps, plus tinsnips, lol.
Here are a couple of tips for you;
1) It is always better to have MORE backspace than you anticipate, because you can always space the wheels to the outboard, as may be needed. But if not enough backspace, then yur kindof fubarred.
2) If you install the max tire height on attempt #1, into a a lowered car, you are just asking for trouble.
3) rolled lips, IMO, look like crap. I suggest instead, to cut the shelf and fold it up. You might be surprised at how much support that shelf actually provides so don't lean on the lip! after cutting it away.
4) marry a skinny wife, and make her work for a living. If she gets to be 140 you will want to mention it to her; don't; just get used to the idea that your sex-life is drawing to an end.