Used GP100 is a better use of your time...
The GP100 6" and larger have mounts for scopes already.
If you really must have a scope mount, the rails I use are from B-Square.
(Had one on my Taurus M66...pal of mine kept bugging me to sell it to him, so I did)
I have a scope on my Ruger Blackhawk 357 from Weaver.