as far as i know it is possible.
In the beginning both the 350 and 500ccm share the same case.
Later on the 500ccm case was somehow reinforced.
You can see this near the rear mounting bolt of the engine
But i suppose this was not the only thing they have done.
The 600ccm case has a bigger hole for the cylinder.
So using a 350ccm case (or a 500ccm) would end up in boring the big hole even bigger to make the cylinder fit.
And you will still have the 'weakest' case of those 3 versions.
So using a 600ccm case might be the better choice.
If i remeber right the rear engine mount bolt is bigger on the 600ccm engines.
So you have to use some kind of thin bushing or make a bigger hole in your frame
What is your plan with the top end?
Remember the big piston would like to have enough air/mixture to make much power