Have you tried removing the rubber disc and replacing it with washers or some other solid disc so the mount cant lift and get at and angle that will allow pressure to be put on the mounts. I am thinking the breakage is from the entire thing moving once the top lifts the strain on the engine side that is solid mounted and snaps the bolts.
That mount is also not strong enough for Heavy duty use. I would do a turnbukle off one of the head bolts.
Grade 8 bolts dont break in line. What i am saying is that a large bolt like that will with stand more than the metal thats holding it if you were just pulling straight against it but since your force is a prying force it can lean the bolt side ways and it will snap right off. Softer bolts will hold up longer. Arp might have something they can recommend that can with stand the lateral force applied.
Did the ends of the bolts come out of the block. Grade 8 bolts can be hard to drill. If you must drill get some good reverse cut drill bits. They should spin the bolts out while your drilling once you take the center out of the bolt.
last silly question did the bolts break on the side pulling up or the side pushing down?
Hope some of this helps. All else fails... 1/4 inch steel plate mount to the front of the motor will keep it in place. This how nascar engines are mounted. Also most purpose built race cars.