Without knowing what the cam specs (overlap/valve timing) is, 15" may be good. A spacer replacing the EGR, assuming all of the possible vacuum holes, leaks and ports are sealed will make no difference for a vacuum reading.

Second - is the vac gage needle steady at 15". If not, you may have other problems?

Next, is the timing correct? It should be in the neighborhood of 9-11degrees initial.

Last, some Holleys had a 'reverse' mixture setting. Depending on the condition of the carb, you could have throttle shaft leaks, body leaks or poor gaskets. Also - a 650CFM Holly is too big for a generally stock 351, especially if it is a '76 motor with the little valves. A 500CFM would be sufficiant.