I have searched on and off for 2 days for an answer instead of asking but geeze!!!
Will a 15X10 with 5.5 backspacing with a 275/60/15 fit the rear wheelwells with out hitting and the car can sit down over them and not jacked up to the moon???

 

No. Not on a 1968-1969. Yes on a 70-72.

Also a 10" wide rim is a poor choice for a 275 60 15 tire.

 

I have done that combo for years, it fills the wheel well better by bulging the sidewall out a little.
Im thinking about bigs and littles but I don't know what I want and don't want to spend money on a back rim and tire combo that doesn't look good.

 

I'm running 275/60/15's on the rear of my 72. Also I only run an 8" rim. Tire fits perfect with no bulge or wide separation at the rim. I would think that 10's are a little wide in my opinion.

 

I'm running 18x9 with a 285/40 tire. Probably pretty darn close to same section width as a 275 on a 10" wide rim. I'm lowered 2 inches, tire tucks in the wheel well no problem. I even removed the springs and cycled the suspension to confirm.

I do agree with the others, I would personally run a 275/60 on an 8" rim, but 10" will work. Too bad 15x9 was never a popular size rim.

 

What I think is terrible is Cragar SS do not come in a 4.5 backspacing!!!