The excellent 45/70 round is 'perfect' in a rifle barrel. However it does have more powder space than is needed or desired [?] for pistol barrels OR reduced loads in rifle barrels. I think what you wind up with in a 45 Silouette is a 45 Win Mag with RIMMED cases, and a .458" bore for a wealth of bullet availability, and a more ideal powder capacity for loads in the hand held gun power range.
A silouette chamber could be cut pretty easily with a boring bar, too. One of the reamer makers should have a Silouette drawing somewhere? All it takes is $$
If we keep talking this up I just might take the plunge. Rimmed cases are desired in the Contender, right?
It's cheap to discuss on the internet anyway!
BTW, I'm not 'new' to this. I built a Ruger 77 Mark II in .458 American, another round 'bettered' for a different setup when the full length magnum round is way more than necessary. The .458 American is a .458 Win Mag cut to 2". About 2 months after completing my .458x2 American, Marlin announced the .450 Marlin round...just about identical to the .458x2 except Marlin wisely designed the case with a longer magnum 'belt' to prevent chambering in other mag rifles.