The CG shown on the manual looks right to me, no reason to deviate from it I'd say.
It would likely be much easier and produce a lighter plane if you used electric power on the Long-EZ. The problem with glow is that you have the heavy engine right at the back and nothing of any significant mass ahead of the CG. This means that you will inevitably have to pile lots of weight in the nose. The real plane of course has a pilot up there to provide 'ballast'. With electric on the other hand the motor is relatively light and the battery can be located forward removing any requirement to add additional weight.

Steve