Its gonna be tough finding that nowadays, I think. Dedicated automotive single stage intermix paints in a good quality, that is. Can't think of one but I have been out of the loop for some time. I don't even know if Deltron can be mixed as single stage anymore but it used to be about as simple as pouring DMD649 instead of DBX695. Maybe some decent brand still uses one set of toners for both S/S and B/C.
I don't know how dark the SPI black is, comparatively, but it would be worth asking about. Next best thing is just use pure black base and clear it, I reckon.
Wheres that bite the bullet smiley when you need it?