The GN400 output shaft is smaller than the LS650
The SV650 output shaft is about the same size diameter.
So the sv gears fit, they were just the wrong configuration to swap the 2 gears out.
There were other differences and appear to be due to the kick ass single power pulses vs. the twin power pulses (beefier engagement pins).
I think the next step is to do some more research. Below I've tabulated the savage tranny. Part No's, and number of teeth. If there are any interested, pick a bike and gather up the p/n's description and # of teeth and post it here. This can be had from the OEM phishes of most online stores. I suspect that the output shaft will have to be similarly sized for it to work. Those that know the chain conversion can chime in to the models that work and we can start from there. Otherwise GS series would be my guess and 650 or larger. Give a shout out if you're in and what model you're gonna do.