Though I do not have a Gibbs conversion, I knew a fellow that did shoot one and he was pleased with it.
There was also an article a few years back about how to extend the chamber to accept .45-90 ammo with no other changes to the rifle.
With some Google time, you could no doubt find the article and other material on the Gibbs .45-70.
Good luck.
CRS, NRA Benefactor Life Member, Whittington Center, TSRA, DWWC, DRSS
Android Reloading Ballistics App at
http://www.xplat.net/
