Edelbrock LD4B ?s

That is a hole someone has added. It was not there originally. Whether you plug it or use it is up to you. I believe the Edelbrock carburetor uses the inside bolt pattern. On the thermostat housing bolts, Make sure the bolt holes are free of broken off bolts and chase them with a tap to clean them out. Then use an appropriate length bolt that will tighten the housing.