Z15 I found the following avail from the (IRONIC) Holley catalog
Base Plate, quadra-jet, bore size: 1-3/8" primary, 2-3/8" secondary, thin stainless steel. PART No. 108-20
I had left mine out and trouble shot the eng for a while before figuring out the metal gasket was missing. 6$ back in 1987. As Steve had mentioned the intake charged is fouled with heat riser gases, Soots up the plugs is one indication, in addition to poor under load performance.
Hope this helps.
Jim
