Nice tank! I've been wanting a 60p for my next tank and hopefully with my birthday, Father's Day, and my wedding anniversary this year I'll get one. lol
Now for your stone support issues... I think the problem you are running into is having too little substrate to hold up the stones. It looks like you have 1/2 inch in the front and 2 inches in the back. I think you can add some more to support it, maybe end up with 2 inches in front and 4 or 5 inches in the back. After that push the stones into position and add more AS around it for more support. Sometimes when I don't want to waste the AS, I add other stones underneath the AS to boost it up. Other people put bags of inert rock under the substrate to raise the slope. Basically do whatever it takes as long as the final support is hidden and doesn't affect the water parameters.
I hope this helps and Happy Scaping!