I agree with mikenash. Serious balancing problem if you're snapping 1/2 bolts, no need to look further. Get a piece of 2" shaft on Amazon and use 38ac's method for your flywheels, then add/subtract weight as needed to tame it. Plenty of these on carts.
I originally mounted mine on a engine room slab, 10" think where the engine was. Even on the wood base and 1" thick rubber pads, it vibrated the whole steel sided engine shed, audibly. I used the Mr X method, which took quite a few hours but did work. On my neighbor's DES 8/1 modified for propane, I used 38ac's method and highly recommend it.
Balance first, then grout as Bob suggests. You might want to unbolt and slip some carpet scraps or such under it for fine tuning the balance.