Has anyone ever stacked a 4 hole spacer over or under an open spacer?
Make any difference?

Yes, no. I’ve never seen any improvement on the time slip from any spacer on any engine I’ve run.

I don't think I would stack one under and open spacer......above maybe but I would probably try one then the other.....I guess you could try the 4 hole on top of the open. My builder had me put an open one on this motor blended to intake.....he has done a lot of dyno work.....but dynos aren't timeslips so I'm not claiming anything.... I haven't tried a back to back. Every motor is different so a lot of variables. Good luck.

Appreciate your answers.