i have just bought a holley street dominator single plane manifold for my 351m-400. it has a hole dead center underneath the carby in the plenum. i wrapped it straight back up after getting it as it will be while before i use it so i haven'y confirmed where it goes. it does not exit the bottom of the manifold, just the false floor. it defininately is original as it looks part of the casting, and some googling shows other manifolds with this hole. what does it do /achieve? also there is some suggestion to block off the exhaust ports on the manifold to keep it cooler, and with LPG it may not be a good idea, but if i do go down this road, what is the best way (not counting welding) to seal off these ports? Cheers.

The hole is for EGR. Seal off the exhaust cross-over passages with brass shim stock. With LPG it is very beneficial to block them off as it reduces intake charge temp.

thats perfect. very logical. so i just position the shim on the intake side of the gasket? about 5 thou do it?