Top tube replacement is not minor surgery.
The way I saw it done long ago was to basically cut the top tube and then remove the down tube from the bottom bracket shell. Then remove the top tube stubs from the lugs, die grinding areas near any thin lug areas first, such as windows. Fortunately, I doubt the Masi has pins.
Then reconstructing the front triangle, this way leaves the lower head lug joint together and some of the top head lug joint. In the example I saw, silver was used to rebuild the frame.
Not a complete step by step, but one basic methodology.
John Jorgensen
Palos Verdes Ca USA