okay build your side pieces right.....get them fine and dandy. get your first floor piece right, then build up off that. jack it up with like a little automotive jack until its level. drill your holes, bolt it down with thread all or whatever that thick *** threaded stuff is called right. then tighten it down really tight. use a breaker bar if you have to get it REALLY tight.
then build your 2x4 supports for the front. you bolt down the back half to the floor, because thats all thats gunna lift in a crash, is the back half, or its all going to slide. no need to bolt the front half down.