Quinn, Yes, the  6/1 flywheel distributes its mass further from the center of the crankshaft where it has the increased leverage (moment of inertia?) necessary to counterweight the 6/1 turning slower. You said:

Since the 10/1 flywheels are smaller in diameter than those of the 6/1 and about the same mass, they will offer less dampening at low speed than will a standard 6/1.  It's also possible that the lower flywheel mass might cause a problem with bearing load at low speeds if the flywheels' kinetic energy is not enough to carry the crankshaft through the compression stroke without slowing excessively.  Remember, none of these was designed to be used for multi-speed operation.

OK if the flywheels are the same mass, but I think the 10/1 stover flywheels might be a bit heavier than the 6/1 spoked. Check this out: The specs (of the brand I will purchase from a reputable dealer) say 6/1 is 800lbs and the 10/1 is 870 lbs. The crate is virtually equal so most of that 70lbs is either on the flywheel or on the crankshaft as counterweights. I think heavier but smaller diameter flywheels and counterweights on the 10/1 crankshaft would equalize it with the lighter larger diameter flywheel on the non-counterweighted 6/1 when they are both turning 650rpm. Perhaps there is someone on the board that can do the math to prove this? I saw somewhere else that you could get a 6/1 with either type of flywheel. Has anyone else ever seen that on an indian site? The 10/1 can certainly be governed to run 800 or 650 rpm, and run smooth, who knows? I would hope we could find a difinitive answer to our discussion. For now, I think multispeed will work if properly done and I will probably try it. If I can ever peal my eyes away from reading all these posts....

The GM 90's are made by http://www.satyajeet.com/de2.htm I remember in one of the threads i read here they were making custom flywheels for a dealer. Perhaps they could probably quote you via email on the "extra heavy" ones you are thinking about making here? just a thought.

I have my 25/2 up and running on diesel.. I also went to the journey-to-forever site and got excited about making biodiesel out of WVO, but it adds time, cost, toxic chemicals (NaOH), etc. but the fuel is excellent. On one hand I want to take good care of my engine. On the other hand, these things are made to tolerate (within reason) alot of abuse. The middle ground (for me) is to get fryer oil from restaurants, filter and use it as-is. Nothing new, many are doing it. You guys might have missed this free site, kind of like Journey to Forever but not as sophisticated: http://www.jatropha.de/ it is about using the Jatropha oil in its natural state (without transesterification) in many types of diesels. check out the page where they fire up a Listeroid in India. an interesting window into a group of countries who are trying to use anything available at the lowest cost.
I can grow this plant where i live so it's tempting. another variety here grows like a weed, no pun intended. Anyway the fryer oil is just too easy. it's virtually free, has no hazardous classifications, and it is working for many. These are the good old days friends. sooner or later our EPA friends will get in the way of our free WVO. Until then.

I would advise you to get 10/1 (still indirect injection) and run it at 650 by changing the governor spring. Ask George about it. Then you have the option to speed it up and change pulleys on your gen head if you need more power in the future or to get you through a tough job. You will be glad you did.