ok guys - help me with my backspacing PLEASE!

I have spent hours searching this forum! I have tried searching keywords also. I give up and I just need your help.
I have a 75 dart sport. I put an A body correct 8 3/4 rear end in it. Now I am in the middle of trying to find the correct rim size for the rear. I want the biggest wheel/tire combo that i can possibly fit without tubbing it or modifying the axle housing. I dont want the tires to stick out past the top of the fender lip. I would like a 15x10 rim. I have tried a 15x10 4'' backspace and it stuck way out past the top of the fender. Now I got a 15x10 5'' backspace and it sits perfectly flush with the outside of the fender - on the top. I don't mind it that much but i think that when i put a tire on this rim it will stick out to far and will probably rub when i hit bumps. This is why i would like it to all fit on the inside of the fender.
Also is there anyone who can tell me what size tires i can fit?
Here is a picture of the 15x10 5'' backspace.

Advise me any way you guys can!
that says 1 3/8''