8 x 68 may not be a good idea for a couple of reasons.

1) It is a rimless round.

2) It is a relitivly high pressure rimless round. (roughly the metric equilivent of a .338 win mag)

I have nothing whatsoever against conversions, but that on a 20G CZ would have me moving to the other end of the range.

In your location 30-40 would be nice and safe.

220 grains @ 2200 is not to be sneezed at. Factory loads are availible for regulation and 30 cal barrels are plentiful even new take offs second hand.

Regards