Using a sbc crank w/a bbc snout should have held up, along w/splayed 4-bolt caps. But at some point, the caps will begin to move, regardless.
The iron LSx blocks are tough- and crossbolted. The LS crank snout is a bit over 1.48" (sbc is about 1.245", bbc is about 1.6”), the wrist pins are bigger, has a bigger main journal size (about 2.56"), even the cam is bigger in diameter.
Stock LS cranks have been used to make more than the power you're aiming for, but a Roots blower will impart more load than NA or even a turbo application, so aftermarket forged is a good insurance policy if it's in the budget.