I was wondering could a 68 rear end fit under my 1967 chevelle with 18's and 9.5 width, would the backspacing be negative or positive?

I have a 66 with a 70 chevelle rearend, yes it will fit no problem. I run 10" wide rims right now with 5 1/2" backspacing. I'm running a 275/50/15. It does rub just a little. I'm putting 17x9.5's on with 5 1/2" backspacing so it shouldnt rub at all then.

Stroked66 do you know if your backspacing is positive or negative? See I want a negative backspacing because from what I understand, negative backspacing gives a deep dish look on rims, as oppose to a positive backspacing. So the other question is will using my stock 54 inch 10 bolt have a deeper look on the rims compared to the 55 inch 10 bolt?

17x9.5 wheel w/5.5" backspacing will have a small amount of positive offset. Running less backspace will give you the deep dish look you want but they won't fit your car.

Clint44, so will it look the same if I used a 55 inch rear end compared to a 54 inch?

The wider the rear,the more backspace you will have to have. More backspacing=less dish,all else being equal.

Clint44 thanks for the information.
I am thinking about instead of 18's going 17 but with 11inch rims. More measurements coming!