What is your elevation? What was the temperature that day? Altimeter setting? Manifold vacuum will vary depending on density altitude (pressure altitude corrected for temp). 15" is what it is for you engine. Use it as a baseline.
Bigger things to look at are EVEN compression across all cylinders and STEADY vacuum at idle.
If you switch to the TQ make sure it is rebuilt and adjusted correctly (good carbs but easily screwed up). Make sure you have the engine running good before the swap, keep the timing the same etc...
If you still have the stock 2" single exhaust don't expect much. If you still have the same cam, don't expect much. If you still have the original cam sprocket don't do anything else until you replace the timing set (good time for a new cam).
That being said, a stock 318 will respond pretty well to a MILD cam, dual exhaust and a 4 barrel.