The bridges won't break unless they were either badly brazed in the first place or developed cracks during use. If it breaks, in my opinion that means you already had problems. The high-end Motos from that era that I have seen are pretty well-built, displaying none of the quality control issues prevalent on their British contemporaries.

It is pretty easy to align a frame with string and a ruler. Having calipers and a large adjustable wrench to align the dropouts and a properly dished wheel to put in and check things also helps.

A Grand Record, as far as I know, should be all Reynolds 531, so it will be harder to spread the rear triangle than the lower-end "hi-tensile" frames, but won't offer nearly as much resistance as more modern bikes would. You can do it incrementally with a threaded rod between the dropouts, but I prefer putting 2x4s or other lumber between the seat tube and the dropout and prying them apart with my arms. This allows you to more easily make adjustments independently on either side, especially if you can fix the bottom bracket in a vise or clamped to a bench. Plus the lumber is easier to remove in order to insert a wheel or whatnot.