The Puma 6 stretcher fit owwww have seen it once in northern ireland for training they set up half the aircraft we tried to put in 3 its bloody hard work and not the easiest enviroment to work in. Also if you set it up that way you loose the flexibility. you can set the chinkook up for a huge number of stretchers but again your looking at dedicated aeromed aircraft which UKPLC hasn't the resources to do so we tend to comprimise with stretcher on floor Puma tail boom is a great place to carry it.
I know the size limitations of the 412 and wouldn't see it as a puma replacement rather as a supplement to the current fleet.
I'm not a big fan of blackhawk and as a casualty the space on the medihawk they're crammed into makes it difficult to do anything if the patients condition deterioates. I was also told by a US medic Col you cant put a NATO standard stretcher on the floor of the Blackawk across the payload area. The Seaking is fairly good in IRT role but again getting casualties in and out is problematic. Chinook and Merlin have the advantage of the tail ramp so walking wounded can walk on and a stretcher can be carried on and not have to be put in like something out of a leaderless command task.