I recently bought a 72 Chevelle and i just happened to notice after looking at the backseat of some other 72s that my rear inner fenders take up the majority of my backseat. Does this mean my car is already modified for a narrow rearend?

Looks stock to me. :)

I don't know. Do you have any pictures of the trunk area?

Yeah I will post it later today, but I have a clearer picture that I will post.

Here is a better picture I will post the trunk this afternoon

It looks like the car has wheel tubs. You need to look under the back of the car and see if the rails are modified as well, they should be. That means the car was set up for an aftermarket suspension and larger tires. Rearend length can be what ever you want based on wheel backspacing.

I guess you have a little problem with that because it makes difficult for backseat passengers to get in and out

Someone already tubbed it for you!! The hard work is done. Now just take your rear seat frame to an interior guy and have him work it a little and it should fit just fine.