Extremely nice find, Dingus! Your best yet, I'd say (from what I've seen so far)... Congratulations!
Now, if you asked me... I'd say that every once in a while a gun comes by that you just can't justify modifying AT ALL (and I happen to love clean & new looking guns, and I just hate rust!). This is one of these guns. If you want to preserve it from further rust, you should put some axel grease inside & outside or something... but a rust removal and re-bluing will make it absolutely lose it's original value, as it will seem like it's something you engraved in your basement. Don't forget that there are ways to tell if an engraving is original through metal aging analysis... but if you remove the rust and reblue the whole deal, it will be much harder or even impossible to do.
This is one of those guns that can very well pay for 10+ new-like 1875's on gunbroker... so why hamper the value by tampering with it at all? In the States, the collectors don't care about how shiny it is.. they care about how original it is (mostly due to their 2nd amendment rights, giving them the opportunity to own & carry whatever they please... not necessarily a shooting antique. In Canada, we care about shinyness and shootability of our "legal antiques", but that's only us... the repressed sheeple!).
Personally, I'd find out what the museum is offering and what conditions they're gonna give you prior to the purchase. I am sure they'll want to examine it or, at the very least, verify the serial numbers. If the gun is modified however (or worse.. reblued!), they will probably just tell you to keep it.
Anyway, that's my 2 cents...
