Daryl - a friend has a full size martini in .22lr, and was looking at converting it to a hornet. I talked him out of it and suggested .50-110.
As for 450/400 - there has been a thread on it, the 3" case wouldn't fit so he went with the 2 3/8" case. Mind you, a .577 2 3/4" will fit.. What caliber range are you thinking/main use?