You may have seen this, but here's the build thread of the guy who did the longhood 912:
'68 Porsche 912 conversion - 2005 STi EJ207 - NASIOC
He had to cut a lot of the crossmember out, but he told me it was because of the STi motor, not the transmission. Since the LS fits without cutting, I suspect you might get away with less work and less modification of the chassis.