"timmholt' - Welcome to the KCA Forum.

You should be able to 'test' the bit of solder that is showing with the tip of an ice-pick or awl. If it is soft, it is Lead Solder. Some local heat with your propane torch will do the job.

Judicious work with some small Files can give you the flat and dovetail 'under cut' to position an S&S Firearms repro front-sight base, like the one you pictured.

Out of curiosity, how long is the remaining barrel on your 'cut-down' Krag?

FWIW - Until you have the replica sight base, I don't see a lot of point in worrying about dimensions. Everything will be related to the finished size of the sight-base.