Is that 1/2" alum plate? I'd use 3/4" min. steel plate for all the havoc between. That will be too much stress on the alum & it will eventually fail. Think about how much wt. you are sandwiching together. Are those 6 - 7/16" fine threaded countersunk bolts? Not enough clamping fasteners & too much mat'l. is removed for those bolt heads - you'll soon see stress cracks there first. Look at any SM plate adaptor & compare it to your design. JMO & I'm just an armchair enjuneer
