Here is what I am currently planning.
Hardwood beams resting on a 1" rubber mat.
The system will not be anchored to the concrete but will be held in location by steel "bumpers" (red) which will be bolted to the concrete floor. The rubber mat seems have the correct natural frequency for this type of weight and vibration.
All experimental at this point so I accept that this may not work.

<click image to enlarge>

Another item to muddy these discussions is that some engines have cast iron pistons and some have aluminum pistons. Throwing a cast iron piston up and down 10 times per second is a lot of reversing force to tame compared to an aluminum version at 1/2 the weight. This may be why some users have bigger issues than others.

Thanks for that explanation BruceM.
I too prefer the high voltage systems but in this case I want the freedom of running the engine at any speed I choose. Also I no longer have access to steel and welders in order to build a new generator base for an AC alternator. So I thought I would try the integral Engine/DC-alternator design.
My GM90 type listeroid is different than yours. The GM90 engines have a counterbalanced crankshaft and the flywheels do not have counterweights cast into the periphery like yours. The Wheels are uniform casings and have been factory balanced with drill holes in the outer ring. How well were they balanced I think I will have to remove them to find out.
If I find that they are unbalanced then there is a good chance that I can use a more simple rigid system after they get balanced.
If they are already balanced quite well, then I will continue to research methods of isolating the thumping.

This picture shows the 20" flywheels (not 23" used on 6/1 Listeroids)
Also you can see the elastomer vibration isolators that do a marginal job of reducing the thumping.

I concur. Using 38AC's balancing method is fast and effective. So much less work and expense than designing a base for a jackhammer.

Please expand on that BruceM. Even a balanced Listeroid has vertical forces from the power strokes.
What mounting method did you use for your balanced machine?

In my case I am mounting just the engine (not a generator frame). My alternator is mounted on the engine.
I could construct a small heavy wood beam frame or a metal frame if having a slightly bigger footprint would aid in the prevention of the "rocking" forces created with bare engine mountings.

Bob, thanks for that explaination.
Just to be clear ... you did not anchor the system to the shop floor?
It's just resting on the rubber isolation pad, correct ?

Nicely done !   

Thanks sailaway,

So, you poured a ~6 inch thick slab and then soft mounted the generator frame to the slab.
How does it perform? Did it control the thumping as you expected?
I would love to see a video of that unit running. 

I'll take some time to study your methodology and calculator.

Cheers

RandyBee1,

The isolators I currently use are made by Karman and were purchased through Acklands/Graiger.
Below is the drawing I made to incorporate the Karman isolator into my engine base design.

Also below is the softer "Navy" isolator I was intending to try on the listeroid.They are from Lord Isolators.

I should note that the Karman isolators were marginal in perforance. They did prevent my garage pad from cracking but they still allowed a thumping force to travel through the ground to my house.
In the case of Changfa type engines, the cylinder is horizontal so the combustion forces are sideways vs. vertical like the lister, so ground thumping is eliminated.

Powdermonkey,
So.., !   I am not the only one who has issues with the ground thumping. 
Here' my short story:
For several years I have used my listeroid which is SOFT mounted to the garage concrete floor located 25 feet from the house.
The generator base rests on 4 vibration isolators which are supposedly rated for the weight of the unit.
I suspect that while they are the correct weight rating, they are not rated for very low frequency power pulses and are better suited to higher rpm multi cylinder engines. I also worked very hard to make the exhaust almost silent so that the neighbors cannot hear the system running. All seemed good. Then last month I was running the engine to heat the greenhouse. When I went inside the house I heard a tapping and bumping sound. I went into the basement and could hear 2 distinct sounds. One was a plumbing pipe vibrating at the same frequency as the engine, and the other sound was a deep bass thump coming through the walls of the foundation. :-(
Remember, this is 25 feet away from the generator. I got concerned that my neighbors may also have this thumping in their foundation.
This is when I got the idea to move the generator to the country to charge batteries at a cabin. So I researched how Lister recommended to mount the engines back in the 60's. I was under the impression that a 1600 pound concrete block would eliminate the ground thump. The ground thumping while trying to sleep at a cabin is also not acceptable.
Your story is very useful and will potentially save a lot of time/effort.

Alternate method:
I did come across some Marine engine vibration isolators which are quite soft and would probably absorb most of the power pulse, however I suspect that the engine may also bounce around a lot. What is your experience?
There are a lot of previous threads relating to Lister mounting and I have read most of them but there doesn't seem to be any one solution that fits all applications.