The length of the cartridge is very important to consider and I doubt the 405win will feed into the chamber. Any 303 based wildcat is perfectly suitable for the Martini, but I am not sure they are the best choice.
My first large Martini was barreled in 45/70. Reamers are readily available and it works very well. I load it to the same level as a Ruger #1 - no problems apart from the recoil in a 6.5lb rifle.
The second rifle is a work in progress and is based on the 348WIN case, any 348WIN based wildcat will work and has a much better case capacity than either the 45/70 or 303Brit cases. I had a new 416 Sprinter barrel sitting around, so that why I went down that path.
My large frame martini is a 450/400 2 3/8 (European 10.3x60)
This sounds like it might be similar to where I have headed, it probably has a bit more taper which would make it easier to load into the chamber.
4D Reamer rental have a 40/348 Ackley Improved reamer available and a 375 version as well. They also have some "Alaskan" wildcats that would be suitable. Unfortunately they will not rent outside the USA.
Luckily Pacific Tool & Die were happy to manufacture reamers for me and their cost was far more reasonable than other suppliers for custom reamers. You have to be patient though, it was about 6mths from first contact to when the reamers arrived. The current project has been chambered using the reamer detailed below.
Red dot sights are the perfect option for Martini actions as it feeds and ejects through the top, and normal scopes don't work well. I wave 2 small, 2 large and 1 Greener Martini.
