Would it be safe to assume that I can use 405 winchester load data as a starting point for the same bullet in a 400 Whelen built on a LR Mauser action.

I'll have to look at my notes, but it is not necessary; there is plenty of 400 data out there.

Sounds like you would be better off looking up loads on the interneat or in reloading manuals for the 405 or any other caliber..bu To answer your question I have no idea..