L98 plenum

Just bought a powder coated plenum. Supposed to have been off a 89. But it is not. I have removed EGR and smog pump from my 89. The plenum and TB are diffrent. Is the extra triangular shaped hole between the TB and plenum used for EGR? If not what is its purpose? Or is it part of the IAC. What will happen if I simply use my TB on this style plenum? Also the runners are set up for cold start. I hopefully can use a plug like a expansion plug for a block if I can find one to fit. If not wher can I buy a blockoff plug. Thanks