Simply put, if you can somehow constrain
all motion and vibration to be oscillitory in the plane of flywheel rotation, you will have a good chance of good results. If you allow the casting to get into a bell-like vibration or let some sort of torsional couple to develop, you will have a very, very good chance of very, very bad results. this goes along with someone else's accoustic energy comment.
Why?
One test to see if a casting is cracked is to suspend it from as few points as possible, tap it, and see that it rings. If you use a high-speed camera to watch a casting ring, you will see quite a lot of distortion in its shape and the relative position of its major features. Imagine what that does to the alignment of a spinning shaft with its bearings. . . . couple that with the gyroscopic effects of a possible out-of-plane oscillation, and look out!
Joining large areas of dis-similar materials tends to damp vibration. . . Hey, look! Concrete, and lots of it!
Cast iron will take a tremendous amount of compressive abuse. It will fail quickly if bent or allowed to flex beyond a very small amount. A casting has low fatigue resistance and fatigue leads to cracks. Concrete exhibits similar characteristics. . . and a photomicrograph of cast iron looks remarkably similar to a cross-section of concrete. (There's probably a lesson in microstructure here someplace. . .) the easy way around this is to add more material. . .
Rigidity is good, but not enough. The support structure has to be even and well distributed. Any experienced mechanic can show you the effects of unequally torquing a head, or worse, a bearing cap) on the main bearing plane of even a stiff cast engine like the 350 GM v8. . . distortion of the bearing bores and alignment will kill an engine fast.
concrete and shims are much cheaper than machined steel slabs. . . and wood is not suitable because it bends, expands, warps and shrinks according to its moisture content. bricks and mortar can't take the vibration the engine would set up. a thin slab of concrete will probably crack in the same way a brick and mortar bed would.
Hence, they make a big deal out of LOTS of concrete because it is the cheapest and most stable of the common building materials.
In summary:
the engineers for your lister intended for it to be fixed well in place to damp the vibrations set up in the casting, and to ensure that the bearings were well supported and kept from moving relative to each other. NVH (noise, vibration, harshness) was not the main concern! the worst thing you can do to your lister (or machine tool) is let it get into a place where the case casting and bearing supports flex unevenly, can move relative to each other, or are unevenly supported. particularly bad is a twist or bend that mis-aligns the bearings (causes rapid main bearing wear and can cause cracking) and the worst is if twist or bend varies as the engine runs, because cast iron has very little fatigue resistance.
If you can make your mount very rigid and keep the high frequency belling in check, and then decouple the very stiff structure from the rest of the universe, I imagine you will do fine.
if you just stick a vibration damping mount under each corner of the engine, I would expect the casting to break or a bearing to fail in fairly short order.
to decouple a big chunk of concrete from the earth, use the dis-similar material idea again: backfill the hole around the concrete block with washed gravel, the more the better. Use some sort of mesh or synthetic cloth to keep sand from infiltrating the gravel, and maybe provide a drain, and off you go. . . pea gravel might work best.
just my 20 millidollars. . . .
-Dan
