Its also possible to find a magic gear for our frame that will let you get perfect chain tension without using the tensioners. just have a look at
http://eehouse.org/fixin/formfmu.php
You are limited with your gear ratio but very doable and alot better than the tensioners. I am just waiting for a new chain ring to sort mine out
good luck with it.
I'd recommend a horizontal dropout like the cross check as mentioned earlier and bypass all this messing with magic gears and such.