Unless you want to run a big big tire, you don't want a 10" wide rim. After reading.. I would think a 15x8" rim 5.5 backspace will be fine for what you want and will not stick out at all. Thirdgens like a 5.5 bs, 4th gens use a 6.5 or 7.5 bs because their axles are wider. I run a 5.5 be on a 4th gen rear axle under my IROC and the tire edges sit flush with the fenders. Yours will be tucked it about 1.5" more than my setup. Ideally I would want 7.5 bs but it is what it is.. if they stuck out it would bother me.

As far as tire size, that 26x11.5 is an Et street, not an ET street radial. They make the radial in the 275/50/15 metric size and you want the radial. It's just as sticky but a lot more stable feeling.

 

Like AutoRoc stated, your looking at bias ply ET Streets. No need for a bias ply! You want exactly as I stated: ET Street Radial.

5.5" backspacing for a 10" wheel will work with a 275/50r15 tire with no modifications to the wheel well or quarter panel. Anything larger than that or any axle width other than stock means you'll have to "massage" the wheel well and/or cut the quarter panel lips. That 6.5" backspacing info you're finding elsewhere? That's for the 93 and later cars with the wider factory axle.

I have pics of my old setup with 275/50r15 tires on the 15x10 wheels somewhere around the house. If I can find them, I'll scan them and post them up so you can see how good it looks.

 

Here is my 4th gen axle IROC with 15x8 5.5bs welds. 275/60/15 Hoosier drag radials. Basically a 28x10" tire. Flush with fender lip. A third gen axle used to tuck them under another 1.5".

http://www.denny.cz28.com/cgi-bin/i/images/ww08_9.jpg