imgoofy,
Gee, what a problem....
I load for cartridges where there's no published loading data every year, and don't always have time to rely on powder companies getting back to me.
One fairly easy solution for a starting point is QuickLoad, but if you don't want to but the program some Campfire members are very willing to generate some QL data if you give 'em the basic info--which does NOT include filling the case all the way to the mouth with water (as you apparently did) but to the base of a seated bullet, which is a far more accurate way to determine capacity.
The other way, which you tried (though evidently not very hard) is to use adjusted data for a similar cartridge. Hodgdon does include 7977 in their 6.5 Weatherby Magnum data. The 6.5 Weatherby holds about 90 grains of water with a typical bullet seated (just about like the 26 Nosler), so I would reduce their powder charges accordingly for the 6.5x68--after you measure its water capacity with a seated bullet. If you don't know how to do that, I can explain.
