I removed the synchro dog teeth from the gears with a carbide insert, welded the rings with soft .035 wire in the MIG. Big trick is skipping around and not letting heat get out of hand. On the ring welded to the side of the 1/2 slider, I used short 3/8" long welds followed by air blast cooling. The weld zone was pretty close to the heat treated dogs and reverse gear teeth, air cooling between short welds kept the dogs themselves cool enough that I could touch them to my cheek.
My thought for using soft weld wire was it would help "absorb" some of the stresses between the two heat treated parts. Not a single problem with the process has surfaced, in the Saginaw application I have even cut dog rings from broken gears and then welded that ring to a different gear. No problems there either.
Grant