To get a 90° corner you need two boards with 45° beveled edges (stating the obvious)
With a 75" thick board you'd need a 1.5" diameter 90° V Bit to cut (in several passes) on the vector line.
To achieve this with a smaller cutter, say a 5" diameter 90° V Bit will require multiple parallel vectors spaced .25" apart with different depth of cuts.
With multiple cuts at different depths to make the V cut there's a good chance of having some sanding to be done.
Hope this makes since.
Honestly some things are better done on a table saw. Just saying.....