Let me chime in to say I also think the existing dies should work.

As for the sizer, the base of the two rounds is exactly the same. Only question is whether it will accommodate the extra 1/4". It probably will, and should be very easy to find out.

The expander and seater will certainly work.

Finally, if you need rid of the short brass, I'm a candidate. It certainly will work in my A. Henry .577 2 3/4" BPE.

Your Martini looks like a nice project and a fun gun. Seems odd that rifles and clothing act in different ways. Your rifle chamber got bigger while in the closet. My clothing seems to shrink while in the closet!

Cheers,
Curl