Really don't what the values are for your area, but the being modified really hurts the re-sale value. The blue book has them going really high about 1700.00 but the last time I seen them at the gun show they were about 40% below book value, but that was 12, the 20 are worth almost double. I would say less that 80%. The book says 1375.00 for a 20g at 60% that also has a lot to do with the year. '66 to '71 have lowest values due to salt wood damage, and are difficult to sell, not sure what the book is referring to but thought you'd want to know.
The Superposed Lighting did not have screw in chokes either, was that also done by a smith. Really don't know if that's possible because the originally a choked barrel is tapered down, not just the end of the barrel. If you really like the gun, what was done to it could help you get a better price but it will be difficult to re-sell. Really a shame they are beautiful guns.
On the other hand it's really great shotgun that would be a good place to start from. Brownings have different feel and fit that most guns don't have, they are known for having short stocks, which is don't work well for me, but I still won't sell it. 750.00 for a quality over and under is low.
Just throwing out some thoughts