I am no expert but on the 450-400 I put together I bored the mono block holes parallel and threaded the same. My thought was that the barrels should be parallel to aid in regulation,it worked for me. Regulation did take about 30 rounds but I had as much vertical correction as I did horizontal, but I regulated at 100yds which may have increased my problems.

As to bushing the firing pins I used a Merkel and the pins where small and tight and I used them as is with out any problems but the 450-400 is a low pressure round.

Over all it was a fun project and only downside is the rifle is to heavy almost 12 pounds.

Good luck Dale