My new home made center sliperential

OK take a look where my project is for now!
I'am realy satisfy to the result.
Here you can see the spur gear adaptor with the stock revo tranny output shaft. You can see the spline on the shaft, and it's the allen screw on the adaptor who'll lock it into the adaptor. But the hole in the adaptor and the diameter of the spline part on the shaft is at "zero", so it's taper. I'll have to heat the adaptor to slip the shaft into it.
The only thing i'am not sure is if i'll lock the allen screw a groove part of the spline, or if i'll make a flat spot on the top of the spline and lock it on this flat spot;



Here is a shoot with the mechanical brake that i'am to install on it.