You've a wealth of information. Given what you have I'd check with Montana Vital records for a death certificate/record. With him being institutionalized they may actually have something that tells you where he is buried.
He is also Listed in Vol I, Montana History as a territorial assemblyman serving in the 8th Session-Extraordinary in April 1873 from Dear Lodge County. [Pg 797 Appendix One]
Another place to look would be local papers for an obituary.