Installing a fiberglass shower in basement
Are you sure there's that much clearance under the shower?  I realize you have a 4" skirt but the floor usually drops back down to finished floor level. 

I would be concerned with those two 90's so close together. you'd never get a snake down there if you had a clog. If you're going through all this work for a shower, take the time to do the drain right and bust up a little concrete. You would fill in the cutout with concrete. The floor of the shower will flex if not supported correctly. Supporting the floor will be easier with a straight down drain.