Time to change out rearends.

Actually with what your talking HP, I wouldn't even bother with the u-bolts on the 7 1/4 that is in there.
Take the 4 nuts off each front leaf spring hanger, then remove the two bolts on each rear leaf spring shackle bracket and roll the whole thing out from under the car. Your going to want to install new XHD leaf springs or SS springs. I'm sure your original front spring hangers have 7/16" bolts and nuts. The new springs will be 1/2", so your going to have to drill the old brackets out to 1/2" or buy new ones with 1/2" bolts and nuts. Your also going to replace the leaf spring bushings in the rear hangers also with new ones.