Firstly that doesn't make it a M39. Secondly it is going to cost more than just buying M39 in the first place (for example I don't think the M39s have the same sight mounting as a M91/30 so that is a major issue). Thirdly it likely won't perform as nicely as a M39.
That being said it is your money and rifle.
Barrels are different diameters on the m39and the 91/30.
Why not just spend the money and get what you want, as m39s arent particularly hard to find.