I'm wondering what your running as a rear end width in an e-body with stock wheel wells. I plan to run Cass's inboard spring kit but I'm cutting down a rearend and thought maybe someone had a recipe I could follow. I tried to run a set of MT et street s/s 275/60/15's on a stock length rearend with 15x8 rims with a 4.5 bs and they rubbed really bad. I don't think a 5.5 bs would help the issue. So I need some help!

Can't answer your question, but narrowing the spring width and extending the axle width both reduce the rear spring's roll rate (but not its compression rate).

I ran that kit with a b-body Dana. 1 inch narrower than e-body on each side. With the same exact wheels you have and a 295/65/15 and it fit absolutely perfectly. I had just about 1" clearance on each side of the tire.

i'm running the inboard spring kit on a stock width housing. with stock wheelwells i can get 325 rubber under there on 10" rims.

The best way is to move the springs, get the wheels and tires you plan to run, and cut the rear to fit. It's the only way things will be prefect.

To add to that, shorter back space makes it easier to get the wheels off. Remember, the wheel hase to come out and to clear the brakes before it comes off.

I guess the issue is I don't know what rims would be ideal. I went with some recommendations and it didn't fit as prescribed. On the drivers side the wheel well comes in quite a bit and rubs the tire. Looking at other pics like Rebel, my car sits lower in the back and I like it. I'm hopeful to have the tire tucked up in the wheel well with this 275/60/15 ET Street.

set the rims and tires in the wheel wells with the rear end removed and get them EXACTLY where you want it and then measure form the backside of the rims to each other and write it down. Then contact a good rear end narrowing shop and have them do the narrowing for you, if it is needed.
New rims are usually cheaper

If you are going to shorten the rear end as stated above the best way to find the rear length is to get the wheel / tire combination that you are going to use. Again the back space is a something to look at as less back space makes it easier to remove the wheel , also a deeper set wheel looks better imo . Maybe you have a friend that has a wheel tire combination that you could borrow to see what might fit. It is harder to figure the tire wheel out than the rest. It could be very costly if you make the wrong decision .Most vendors will not take a rim or tire back once it has been mounted. Also keep in mind to do any of this the rear must be removed.

This is exactly what I did and my car acme out just like I wanted it. I also agree with using the minimum back space, it makes getting the tire/wheel off easier.

I made a fixture of angle iron, a long narrow rectangle to set the tires and wheels in. With the car on jack stands I raised the tire/wheel up and slid the fixture under them and used a floor jack on each end raised the tires and wheels up into the wheel wells. Then it was just a matter of centering them in the wheel well and measuring for the rear end. Remember to subtract brake drum or rotor thickness from the measurement.

I have a B Body diff with Cass's 1" rear spring relocators, 15x10 rims with 5.5" bs, 295's on rear, with car at normal ride height (tires up in the guards a little) its tight but no rubbing with no fender lip removal or toll.

I can't remember the backspace on my wheels right now so when I get home I'll see. I used the b-body Dana and 12 inch welds and M/T sportsman tire 29" tall, I think they're 14-15 on the tread, of course the frame is moved in and original tubs are widened 5 inches. New box frame back from the trans crossmember. Fits real nice up in the well.
I'll be back