Auto-X / Roadrace rear suspension 1969 Valiant

1.67 degree angle from the E body in @72bluNblu 's post equates to about 1.66" of taper (per side) with a 57" spring (which seems to be E body length). The amount of taper is about .03" per inch: 0.291 at 10" of length.

If you want 1.5" of taper over a 57" spring, the spring toe angle would be 1.507 degrees
If you want 1.5" of taper over a 55" spring (a body), the spring toe angle would be 1.562 degrees

If you are going to attempt to taper the springs at all, I would try to match the relocation box and sliders to the angle of the springs. Doesn't take much angle to subtend some measurable distances.