Open spacer feeding into a split plenum. I do have a 4 hole 1" spacer that I plan on putting in next month when I do the transmission swap from auto to 5 speed.
I've read hit a miss with TBI and dual planes with no balance port between the plenum divider. Holley just says to use the provided gasket but there is no picture of it. I'm not saying this is or could be your issue but worth looking into.
Have you data logging to see what you AFR is at idle, cruise and WOT is?
