I have a Pedersoli in 45-70 & am following this thread with interest.In the case of my rifle the only need I can come up with is more powder capacity to push bullets above 4oo grains fast enough to reach regulation velocity without excessive pressure.My rifle will shoot up to 425 grain cast at regulation velocity-1770 FPS.I can't see enough gain to go with rechambering to something bigger,
even 45-90 or similar Cal. because of anticipated regulation problems with heavier bullets than what works now.Mine shoots any weight from 300 to 400 plus pretty good but the heavier they get the tighter the regulation velocity gets.300 grainers have a range of plus or minus 60-70 FPS while
the 425's have a window of only about plus or minus 20 FPS for good regulation.
I believe that the advice given above about a 9.3 x 74R or even a different model would be money much better spent than the allmost certain regulation problems you would get with a rechamber.If the truth is known the 45-70 will handle about anything this side of Alaska & Africa except the ones
that bite and is better than a big club even then.Find what works in the 45-70 and respect its limits.

Big Mike