As above, thank you for serving our country. Please stay safe and come back in one piece.
Just to be clear, most SP89 pistols were imported after the '89 Non Sporting Use ban, which is still in effect. I'd suggest you look up 922(R). If your SP89 was imported after '89 then 922(R) would apply. My understanding is any "non sporting use" rifle made from a post '89 made or imported receiver should have no more than 10 foreign made parts out of the listed parts. It is a felony, so I take it very seriously, even though the odds of being caught are very low. Part of the SP89 appeal is all German parts, so properly converting the SP89 would loose some of its collector value. Once the SP89 is converted to a rifle, my understanding is it can't go back to being a pistol.
Mass law governing SBRs requires that you get a machinegun license through the Chef of Police of the town you live in. These are very difficult to obtain. Politically Mass. is very anti gun. There are lots of towns that the Chef won't issue License to Carry permits. If you want a carry permit in one of those towns, you have to take the Chef to court. Then take them back to court for every renewal. Five years ago there were only a little over 1,000 machinegun licenses in the whole state. They are issued at the digression of the Chef. All LEO that use a NFA weapon in the course of their duties have this license. So there are very few issued to collectors. If you are going to live in Boston proper, they don't even allow Class A (CCW) permit holes over 10 rds in the mags, even if the mags are pre ban. BTW Mass still has a state law that was fashioned after the National Assault Weapons Ban. No new mags over 10 rds (must be pre '94). There is also limitations on handguns. It would not be easy, if even possible to take a SBR with you to Boston. Again, stay safe. Good luck with the project.
Scott