I viewed the pics. and think you may be able to support that. When I set tubs,wirlpools and shower bases I use Gypsom plaster. Home Depot usually has it and you can find it by the drywall supply section. Get a mixing padel 5gal bucket and a drill (i use my right angle drill) that with stand low speeds with a lot of torque. Mix it and fill in all the gaps under tub, when it sets up it will be very solid. Make sure when you mix it you have a thicker consistency so it does not just slide off the padel. Hope this helps.