First thing, make sure the rear sprocket and the front sprocket line up. I had to put a bunch of washers to move the wheel to the left to make everything line up right. If everything does line up right you might not even need a tensioner if you get half links and play with the chain size a bit. Unfortunately on my bike I need a tensioner more to keep the chain up from hitting the bike. As 2door said, the first thing I had to do was bend the tensioner to get it to line up with the chain.
First I used the stock roller, but ditched it after a few miles because it is crap...
Then I use a derailer gear which actually worked pretty well, but for some reason it was eaten in about 100 miles. I think because the stock chain is really crappy and the rollers stick, and sticky rollers ate the gear for breakfast.
I have now settled on a skateboard wheel. I used a high speed drill as a lathe, and a file to cut the groove. And it works mint, couldn't be happier.
And I'll second what 2door said about the tensioner being wider than the chain not being an issue. The chain tensioner is not for guiding the chain onto the rear sprocket because it isn't aligned properly. In theory you should be able to use a infinitely wide tensioner sprocket, and still have the chain properly rain onto the sprocket.
I did actually try to use a derailer as a spring loaded tensioner, but I never got it to work right, so I gave up.
I'm also running a KMC420 chain. A bit wider than 415, but the same pitch and hasn't caused me any issues. And it only cost me $16 for a 6 foot bit.
