Put it on and give it a shot. You'll just need to leave the rakers a bit higher.I was going thru my bars and chains and found a 3/8 small frame that will fit the 550XP. The only problem is that it is a 20 inch setup. I also found 2 18 inch bars but they are both .325 spockets.
As for the 455; I'd just swap the tank and run the cylinder missing the fin. As long as you have enough a sharp chain and good mix you should never have a problem with it getting hot, and you'd have a spare p&c if every needed.