Mechanical temp gauge question

The large port you're talking about is the one for the power brake booster. That will not help you with temperature. It needs to be at the front of the manifold near the thermostat housing. Most aftermarket intakes have a plugged hole in that region that is larger than the one for the stock sensor.

If you had the newer edelbrock performer manifold instead of the LD4B you'd be set, there's only about 4 holes up in the front where they need to be.