Picturing that situation makes me think of a high perch tub and slab with steep slope on one or more sides. I guess the over-size deck on the slab sounds like it might be best but I caution anyone on exact dimensions up front. Do Nothing until you confirm the base size. Even if the exact tub isn't local or "on the floor" at a dealer, the ref size info on another model of same brand as 80 x 80 should be a twin and offer the base details you need to confirm. Best of luck and keep us posted on the project and outcome !