Any combination of the 3 options below will fit with room to spare according to our wheel and sizing tire guide. While all of them can be run square, I will never need to rotate the tires, and I think square looks a bit odd when the fronts are flush and the rears are tucked.

So I am leaning towards the 9" +50 for the front, and 9.5" +50 for the rear. While this will still be far from the limits, it will be a big improvement from stock and the +50 will keep the suspension geometry identical, while still giving a very slight stagger. I was planning on 255/35R18 front and 265/35R18 rear. It looks like 265 is no longer an issue as there are plenty of tires in this size currently. Since it is such a small stagger, I don't think it will cause any noticeable increase in understeer. It would be nice if there was a 9.5" +45 option, but I figure I can add a 5mm spacer in the back on the stock bolts to fill out the rear fender a bit more if it needs it.

My plan is to put the original wheels and tires back on when it is time to sell the car, and I'll sell the BBS. Resale of the BBS is a slight concern as well as these are not cheap wheels, but I doubt there is much difference between these sizes other than square is probably easier to sell.

 

What are you looking to do with the car? Is it lowered? What is your camber at?

Assuming you want maximum performance, and those are you only wheel options, I think I would go with 255s all around on the 9.5s.

Use spacers to achieve the look and handling balance you want.

You can also consider the 10.0 or 11.0 +50 options if you've got a lot of camber and want to stuff some fat rubber under pulled fenders.

 

It is primarily a show/collector car with 14k miles, which is why I'll never need to rotate the tires, just replace them as they age out every 4-6 years. It's also why I want to be super conservative and not even take the chance of rubbing. However, I still want to drive it like it was intended to be driven for the few hundred miles I drive a year. There are some fun roads around here in Texas Hill Country, and also CoTA, but it's hell on an unprepped car. I'd have to tape her up, swap pads and fluid, and then still take it very easy to not cook all the 30+ year old parts into oblivion. I am sure it is would be incredibly fun, but probably not worth the risk.

With an entirely stock power train and suspension, 255mm of modern rubber is already way more grip than the car needs and I won't be going for any track records. You simply have to go that wide to fill up those massive wheel wells while keeping the geometry in tact.

255 on the 9.5" all around is a great option. It would clear the front fenders more comfortably than a 265 and provide the neutral handling that is well praised on this forum. I suppose I could still put a 265 on the rear for a some tire-only stagger, but 5mm is the max spacer I would put on stock bolts.

BBS does offer the RE-V7 with the perfectly staggered 18x9 +45 and 18x9.5 +38, so both front and rear would be at the minimum fender clearance according to the XLS. The reduced offset on the rears is not as much of an issue than the fronts since you don't have to worry about scrub radius but it will add stress on the wheel bearing and I've had my share of wheel bearing issues. Plus I like the look of the RI-A better.

So I think the 9.5" +50 on the rear is a given. And it is either the 9.0" +50 or the same 9.5" on the front. Front tires will be 255 either way. Rears could be either 255 or 265.

 

Before I rolled my fenders I had
Front: 18x8.5 +43 ran both 225/40 and 235/40 tires
Rear: 18x9.5 +40 ran both 255/35 and 265/35 tires
Never had an issue. I did have a bit of sunken battleship look though. I hated it so ultimately I rolled the fenders and added +15mm spacer to all 4 sides.

On the available sizes by BBS on the RI-A wheels, I'd personally would op for
Front: 8.5 +40 with a 225/40 tire
Rear: 9.5 +35 with a 255/35 tire (I'd triple check this one though)

But from the choices you have given I would do:
Front: 9 +50 235/40 tire <-- this offset is not bad.
Rear: 9.5 +50 255/35 tire <-- holy sunken battleship Batman. I'd for sure go with a 10mm spacer to match my old setup on the rear since they never once rubbed.

I was thinking about a 255/265 front/rear combo but 255 seems a little meaty for up front. I have a feeling that might cause rubbing issues especially if lowered (it's just a feeling so take that very lightly).