I will yell you right now what will happen, because when I was first playing with slants I mistakenly ran the distributor vacuum advance off the manifold thinking it would improve performance. Your base timing will need to be seriously retarded to compensate for the fact that the distributor will be running full vacuum advance at idle. As soon as you step on the gas the timing will retard because it will lose all the vacuum advance. The mechanical advance will start to come in around 1200 RPM (depending on how the distributor is set up), but in the meantime your timing will still be retarded, possibly past TDC, due to your having to compensate for fully vacuum advance at idle. Your fuel economy will nose dive, your performance will go in the toilet, and you will likely have to make the carb run very rich to compensate for the poor timing performance. The only times your timing will be close to optimal will be at idle and at light throttle cruise. Otherwise you won't have the benefit of vacuum advance and your timing will be screwed up due to the required retarding to correct the base timing at idle.

But feel free to test my hypothesis in a scientific manner. Run the vacuum advance each way (ported and manifold) for five tankfuls and document your economy. If possible, take the car to a track or use a smartphone dyne app like Dynolicious to document differences in acceleration and top speed.

Report your findings.

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Casually looking for a Clifford hyperpak intake for cheap.