Rear End

I believe my 72 duster has a 7 and 1/4 rear end but my dad thinks it will not be strong enough. I recently purchased eldebrock performance rpm heads, comp cams cam and lifters, eldebrock performance rocker arms, comp cam pushrods, eldebrock airgap intake, msd distributor and racing rotor, as well as a msd coil. I am purchasing a MSD fuel injection system as well. My car came Stock with a 318 so I was hoping you guys could help me out with the correct rear end choice. I appreciate it in advance!:burnout: