Even if you cut small access ports just where the tabs are in the floor pan the crossmember is wider then the space between rockers. The only way of getting by this I see is either cut the tabs off and push it up in place from below.

Or possibly bend tabs of new crossmember up vertically, push member up from below, hammer tabs down over rocker.

I do kind of like the thought that at least if you cut the access ports right over the flange of the crossmember you can stitch them shut instead of trying to butt weld.