1971 cutlass s frame ?

[corvette427]

I just got a cutlass S given to me by my cousin not really but long story, anyway im going to build a pro-touring car out of it. based on w30 442 clone. ive been reading the forums and i know that i need to box the frame so ive been looking at hpi and others but i like the extra bracing that their kit offers. im going to do full tubular a arms up front with coil over conversion all around tubular control arms in the rear with abc mini tub kit... i would like to be able to put some 345's on the rear. how does the fully boxed oem frames compare to the aftermarkets as far as rigidity? i think that maybe this only cost a fraction of the aftermarket frame