I would go "small" (relatively speaking of course) on the intake and cam, since the heads are going to be in no way a bottleneck on a 306.
The guys are correct about the short stroke in these engines LOVING to rev.
You can spin the hell out of a 302 based engine (289 even more so), and not have an issue.
I think more than the rpm you are worried about, you need to worry more about splitting the block.
302 roller blocks are known for falling apart around 400-500 hp, while stock Windsor blocks have endured 700-800 hp before splitting.
In short, don't worry about going over 6500 rpm in a 302, that's nothing for a short stroke.... be worried about the block not holding up to your future power adder IMO.