Couldn't you just hole saw the splined part out of both wheels and weld the old one into the 08 wheel? That's how I would do it. An adapter will make an unsightly gap.
I had this thought, but I'm afraid it wont be that simple. I'm actually least worried about the splined vs hexagon steering shaft differences.
The back of the steering wheels are both very different.
Here is a 99-07 SD Wheel:
Here is a 08+ Wheel:
It appears that in the 99-07 steering wheels the clock spring recedes into the steering wheel to produce a flush fit.
Whereas the the 08+ steering wheels have a clock spring that recede into steering column itself to produce a flush fit.
So I've had two thoughts.
1.) Cut out the back of the 08+ steering wheel similar to the 99-97 wheels. With the back cut out, redesign the entire metal brace inside the center of the steering wheel using a CNC machine.
2.) Create an adapter from aluminum and 3D print new plastic column cover pieces to produce a flush finish.
Either way I'd like to eventually sell kits so people can easily upgrade to the newer style steering wheel.