The reason we got a different gearbox than the e46 one is because that one didn't fit. I wouldn't expect you to be able to fit it for a reasonable cost if BMW gave up on the idea.
The only real thing that helps the grinding so far is the
vibra technics engine mounts.
I'm not convinced stiffer transmission mounts help that much, but I don't have them installed so I cannot say for certain.
I don't think SSK helps either, I do have that installed (and it's worth every penny, just not for fixing grind).
I'm hoping that vibra-technics makes us a transmission brace. That + engine mounts has a real possibility of fixing the grind (or at least making it so you have to slam the transmission into second to have a problem). Read the thread I linked for more info.
Other than that, search the forum. This topic has been covered 5 million times. The current best answer is what Rob says above: learn how to do a heartbeat pause between 1st and 2nd.