I am starting on my first double rifle project and would appreciate your opinion on a few issues

I have bought a 16 gauge Simson and Suhl (Thuringen) from 1967 that apart from some exterior rust on one side of the barrel is in new condition. It dosn´t look as though it has ever fired a shot! It cost me aprox. 60 US.

Questions; calibre – I was thinking of something like 450/400, 450 NE or 470 NE. Ellis brown states in his book that 450/400 is to heavy on the recoil due to the light weight of the finished gun.
Having shot 338 RM, 375 HH and 416 RM for some years I am used to magnum rifles, so recoil dosn´t frighten me. Any suggestions?

Having only a mill and not a lathe I had planned to order the barrels finished from Lothar Walther, Germany. This leaves me with a problem; if I get the barrels finished I will not have the reamer to cut the chambers and will not be able to adjust the headspace by cutting the chamber. I was hoping to get by this by ordering the shoulder on the breech a little shorter than the monoblock, I could them adjust the headspace by taking of material on the face of the monoblock.

Thankful for any comments

Regards