In my late teens I inherited a '62 Impala, and it's long gone but I am familiar with your problem.
I just put in a BendPak 4-post lift and a single
bridge jack, but several members here have also purchased a second bridge jack. I am fairly certain that with two bridge jacks, considering their adjustable arms and ability to position anywhere front-to-back, you could easily handle your x-frame toys.
In my research, it seems that normal 3000psi concrete at any thickness over 3.5" can handle a 4-post and its load in the sub-9000lb range. If you plan to have the lift in a fixed position, you could beef up the slab by creating some "beams" with a little extra depth in some channels in the fill/earth and an extra stick of re-bar or two. Wouldn't cost much more and you'd have no cause for worry with just about any lift.