Sorry for the big red X, but you can't compare the 8.25" you have in your car to an 8 3/4 with BBP. Totally different backspace needs. The 7.25/8.25 rear ends are narrower by just over a full 1" compared to an A-body 8 3/4 with BBP axles. Changes the backspace by the fat end of ~1/2" per side.
First, is your WMS to quarter 5"? Because if you've got 6" from WMS to spring and 11" total you need 5" from WMS to quarter (also would match my Dart's measurements).
There are several people on this board running 255's with stock spring locations and quarters (
@toolmanmike ), so, I wouldn't say you need the offset springs to get 255's. But 255's would be pretty easy with a 1/2" offset. Some folks have run 275's with just a 1/2" offset on the springs. Running 18's helps both of those, because you don't need as much clearance because the sidewalls are short and can't move around as much as with a taller sidewall.
A rear disk set up only adds the difference between the thickness of the drum brake at the axle flange and the rotor thickness at the axle flange, so more like 3/16" to 1/4" for width.
Some basic things, if you're looking at a 9" wide rim that wheel is actually 10" wide outside lip to outside lip. So, with a 245 on there at 9.8" at the section the rim itself is basically the widest part, you don't need to leave much if anything for tire overhanging the rim if that's your tire/wheel combination. The other thing is that you're running 18's, so, you will have a pretty short sidewall. You won't need a full 1/2" of clearance between your tire and your springs like with say a 60 series tire on a 15" rim. I've been down to about 5/16" at one point before I added my rear disks, I never had an issue with tire clearance on the springs, only the tire to the inner fender (also with a 1/2" offset).
So, for your math, if you've got a 18x9" on there and are going to run a 245/40/18 you're going to have almost no tire overhanging the rim. Even a 1/2" of clearance to the spring means your backspace could be more like 5.5". You'd lose a smidge because there will be a little sidewall on the other side of the rim lip, but it's gonna be VERY minimal. On that note, I think you'd be fine with a 255/40/18. Most of those spec at about 10.2" on the section width on a 9" wide rim, which means you've only got like .1" hanging over the rim lip itself. So, you spec 5.5" for backspace, you lose a tenth to the tire, you've still got more than 3/8" to the springs and you're fine. On the front side you've got 4.5" for the rim to the outside lip, a tenth for the tire and .4" for clearance. And you can see that even that won't leave room for your disks conversion down the line. So more like you spec 5.75" for the backspace and run a 1/4" spacer until you install the rear disks, and everything stays centered.
Tight with the 255's for sure, but again, there are guys on this board running 255/60/15's on the back of uncut and unmodified Darts. That's a 27" tall tire too, which brings up other things. The clearance needs on those tires are quite a bit more than for a set of 255/40/18's, just because the sidewalls for those 18's are not going to move around much, if at all.
