What brand of chain mortiser? The Makita will allow an angled cut. The cross cut types, however do not.
In using hand tools, I have used a boring machine set to the angle and bored the end hole at the angle. I have also used a hand rip saw to cut each side of the mortice and finished it with corner and regular chisels. I have also used the Stihl method.
Perform most of the work from the wide end of the mortice, try not to push the last bit out the other side. Have good crisp ends marked on the mortice ends. I work the narrow end first and only an inch or less into the mortice, to lessen the chance of flipping the face out. Use the framing square to ensure both ends of the slope are true. It should be as true as you can make it, for sure no humps in the slope. I use a 1-1/2" chisel on 2" mortices almost all the time now. It is much easier to push/pound.
Is the cross beam housed into the 8x8 post? If so use a haunched shoulder not a full square housing. A square housing makes for difficult assembly.
I like half dove tails. But it could be reasonable to just use a through tenon with a couple heavy pegs. Span and loads would impact this decision.
Are you using a wedge driven from the inside or outside? I drive them wicked hard from the outside and then put a fastener in the wedge, screw or nail. I don't want it to move after things dry.
