if i where you i would definatly not put in a barrel longer than 550 mm with a bore up kit. you may and i say may be able to run the 590 mm barrel with only a slight loss in FPS, but once you hit a 650 mm barrel your FPS will suffer a lot, to the point where a M140 spring will perform as a M120 spring.
if i where building your rifle i would snatch one of those 509 mm tanio koba twist barrels, according to different tests i know where performed by experinece people they are very nice also i a M170 powerde gun as well.
i would go with the systema bore kit if i was you, reliable and not too much noise. 7mm bearings are much more durable than 6mm, it does after all usuallty cost a new gear set if they blow so why risk it. the best 7mm bearings can be found here
http://www.kanzen-bearings.com. they are super nice, i use them in a M170 setup and they have never failed me. and i run it Full auto frequently, albiet only at 700 RPM