That depends a lot on what you are doing with the Jeep. With 37" tires, 100" is a pretty versatile wheelbase, but If climbing big ledges or other extremely steep obstacles is your thing, you will want to be more in the 105-110" range. If maneuverability and break-over angle is important (tight wooded trails, technical boulder sections) sub 100" wheelbase is really nice. 100-105" is a nice compromise between the two generally.
Basically it is a trade off between maneuverability and stability, as well as approach/departure and break-over angle.
By stock body, I assume you mean that you don't want to comp cut the rear? If that is the case, 4-5" is pretty much the limit of how far you can push the rear back. You will need to address the gas tank. I know Genright and probably a few others make "stretch" tanks, which are essentially reduced capacity tanks that mount in the stock location and are designed to clear your rear axle once it has been relocated.
By the way, 4-5" of rear stretch will put you at 98-99" of wheel base. I have a rig on 38s with 99" of wheel base, and it is noticeably more stable than the original 94" wheel base, although it is hard to say how much of that is due to the full width axles. I am planning on going to 105" in the future though because I like to go up big ledges.