Conical bushes are not a panacea for all pens, be careful not to flare the tubes ends, then you'l be complaining about split wood and ill fitting finials etc.
They do work for applying ca or as an emergency bush for a makeshift or experimental pen.
How big are your cones going to be? Some tubes will swallow the cones inside on the larger kits.
The only real answers have already been said, either use the right size bushes or don't use them at all.
Above all measure everything and act accordingly.