[1project2many]

First, to answer your question, the vacuum pump should be Teed into the line from the manifold. You need a check valve between the manifold and the Tee to prevent the pump from continually drawing air and vapor from the manifold. Use a vacuum switch to operate the pump. The vacuum switch should sense vacuum in the booster, not in the supply line. The booster will most likely already have a check valve in the inlet fitting. This is OK to leave in place as a safety measure.

Second, 18" of vacuum should be enough to provide plenty of assist for at least one stop. If the engine idles at 18" the booster should also be at 18." Additionally the check valve on the booster should keep the vacuum level at 18" even when manifold vacuum drops. As a further test if you drive the vehicle in low gear with the engine at 3000-3500 RPM, then close the throttle to decelerate, you will generate maximum vacuum in the booster. Vacuum should easily climb above 23" at this point and again the check valve should hold the vacuum level in the booster at maximum. If you do this, and you cannot get the brake pedal feel and brake system response you are looking for during the very next stop, then a vacuum pump will not help.

I realize you may have checked all of this but I am putting it out for consideration anyway. I fear you may be heading for an HB unnecessarily and you may also be dissatisfied with the result of the swap. HB systems actually reduce the mechanical advantage between master cylinder and wheel cylinders / calipers by using a larger master cylinder piston. They overcome some of this with extra hydraulic assist but IME a system which requires "too much" pedal pressure in vacuum still doesn't feel right if switched to HB.

HTH