The 350 rear end swap is the way to go. I did it with mine.
If/when you find hubs just be sure the splines on the insides are good and square. all too often you will find ebay sellers will not disclose that information and your left with a useless hub. In our case with the swapped rear ends we need new old stock or good used condition hubs because they are getting so hard to find.
The 350 rear end is better built in every way over the 300. you wont miss the old one. Be sure when you do the swap you properly set your shock location or you will either gain or lose seat height.
also, not much is mentioned about brake cable setup. you will find you will have to modify your brake cable perch to get functional brakes. not a big deal, but to get your rear brakes to work, its not a simple plug and play, it will take a little bit of re-engineering.