Sounds like a good plan!
If you can do it safely, check timing at 650 rpm. IMO 14 at 650 rpm would be close, and Rob suggested 15* would be good.
You may find a huge diffence in timing from 975 to 650. Only way to know will be to measure.
And don't worry about the jetting/rod for now. As long as the rod/jet combo is significantly larger than the idle restrictions, it will have no effect on the idle and off-idle.
Carter idle system is a little different than the typical Holley in that it uses a second air bleed and restrictor. So general rules of thumb for a Holley about 'square' or 20-40 thous. slot exposure don't apply. However the concept is still valid: More t-slot will provide richer idle. Too much or too little will cause flat spot or delay.