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I'd suggest the larger secondary as well as otherwise you'd have vignetting.
If I understand correctly, the increase to the central obstruction would only be an issue if its for planetary work.
What is the weight so far and how are you mounting the primary?
Keeping the focuser as close as possible to the rings without the drawtube protruding too much is ideal.
so focus with a dslr could be say 5mm from fully racked in. Focal point for CCD's would be further outward as distance to body is a lot lower than dslr's.
Cheers
Alistair
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