Can either a 500 Jeffery or a 505 Gibbs be built on a Montana 1999 action originally intended for a 375 H&H ?

Yes, it probably can be done but will take quite a bit of work. I'm in the process of building myself a .416 Rigby on my Montana 99 and I've got the mods done so it will feed. I've started to make a quarter-rib and a stock pattern for it but those have taken a backseat to paying jobs lately.