Hi
Personally, if you were on a budget, and just didn't have the funds to install an 8" rear end at this time, I think you could live with the 7.5 rear for the time being. The car is very light and would be more likely to spin the tires. besides, when installing an 8" with a 5 lugs, then you'll be lead to changing out all your suspension and probably do a drum to disc conversion. but you will probably be having to upgrade your front springs for the additional weight of the 8 cylinder motor.
BTW, your car looks great, the 6 cylinder also looks great! Why are you wanting to go to an 8 cylinder?
dne'