First off....
you'll want to look for one like this...
http://whtractor.15.forumer.com/index.php?showtopic=1363
This one is a 36". They were also made in 32" versions... I would suggest, with the 6hp, that you try to find a 32".... BUT, you should be able to get by with a 36" IF you don't wait too long between mowings...
Where are you located? I have about 5 of this style deck.... Also, you can usually pick them up on e-bay CHEAP... Hope this helps, Jon