[SamUK]

I've done this mod recently and it took me a day. The 350D rear end bolts straight up with no issues. You'll need a welder and to be relatively knowledgeable with light fabrication.

Cut off twin shock mounts, weld new one on for your single shock, weld new swing arm mount to 350D rear end etc etc

Follow the link above, I came here for help on this mod and it helped a lot, some very knowledgeable people on here who are more than willing to help.

 

[SamUK]

I'll add some pictures later of how I did mine. Very similar to fishfiles idea but I sleeved mine with tube rather than using the 300 centre tube. Only because parts here in the UK are expensive and I didn't want to chop it up.
My brakes, I used standard 300 cables and modded the brake cable mount on the 350 axle. As for tyres mine are 10" wide and I use the 300 fourtrax wheels as it gave me a little more width, they just about clear the muffler, guessing that's down to tyre choice.

 

[SamUK]

Haha.. Yeh it seems spares over in the USA are plentiful compared to here in the UK fish. For a good rear end you'd expect to pay around £300 here. 350D's are very hard to come by here now especially spares. That's the only reason I didn't cut up what I had as my dad has a few 300's and may need parts off my old rear end.

You'll be able to start a museum of Hondas through the ages soon with that many ATV's fish haha.

Dan.. before paying for repairs look on here and get talking, you may find somebody local to you who is willing to do an axle change or mod for you, or even have a spare axle to sell you.