i would add one more critical component to the equation
that being placement of the isolators,
in my opinion they are in the wrong place as constructed
the ones under the engine need to be further out by maybe as much as a foot
and wider by nearly the same,,
this from what i can see in the pictures
where they are located they have very poor mechanical advantage or control of the vibrational and torque action applied to them
i spent a hell of alot of time argueing with GuyF about the possibility of using resilient mounts,,, and it took me a long time
to understand his concerns.
that being
if you want to build a resilient mount system, you must engineer it, if you don't your results will range from possibly good if you are extremely lucky
to absolutely dangerous and everywhere in between.
you setup is clearly somewhere between the two extremes.
time now to go back and engineer the system,, and see if what you have can be modified or if you will need to scrap it and start over again.
if i might suggest go back and look at how the oem lister was mounted to the block of concrete,, particularly the early version where ~625 lbs of concrete
was used. print out the drawings and use a straight edge to see how the vector lines work out
after doing so you will see where those rubber mounts should be placed.
this all assumes that the steel subframe assembly is very rigid! any flex and all bets are off, all your engineering will be off
because the flexing frame becomes part of the resilient nature of the mounting,, and there is no easy way to calculate
the action of a flexing frame and its contribution to your problem.
as much as i hate to admit it GuyF in many ways was and is correct, what looks good or appears to be intuitive often times just don't work!
your frame assembly while looking good is an example of what he was illustrating in 10k words or more
bob g