The 6CM and 6mm (243) are extremely close in case capacity. IMR4064 is hard to beat in the 243, and it would be a good powder for your application. With the lack of available data, I would start with published loads for the 85/243 combination ON THE LOW END and work up slowly watching for pressure signs as you progress. Go slow and in .3 increments, because while this seems a reasonable approach it is still "no man's land" without any firm data to work off of. If IMR4064 isn't available, H4350 would also be a good choice. Of course, use the H4350 data in that case.
There's a fine line between BRAVE ... and STUPID.