IMHO, you will not have much choice, the reamer will follow the hole as bored in the monoblock unless you do it with a milling machine and a cutter rather than a reamer. Reamers are not designed to do anything other than follow the bored hole.
I would doubt that at the breach end the barrels are anything other than paralel. With mine after regulation the barrels ( bores) are only slightly less than paralel.
Again IMHO I would in setting up the barrels for regulation make them .025 less than paralell to start with.

Regarding balance, if you make up a pivot and set the gun on it about the hinge pin and add clay or the like to the butt till it balances, now you can tell if you can balance the gun with some lead in the stock without making it too heavy.

Regards