rear disc conversion

Remember that Changes in camshaft lift/duration/overlap move the powerband rpm.
Street cams were designed for street rpm driving giving maximum vacuum= max fuel efficiency.
The higher up in the rpm band your camshaft is designed for, the less efficient it will be at lower rpms.= poor /erratic vacuum. That it the rumpity rump you hear at low rpms that goes away as the rpms increase.
The reason that high rpm cammed engines need to be constantly revved up at idle is to clear the unburnt fuel from the plugs and combustion chamber, not just for the cool sounds.
Everything is a compromise.
The closer you try to build your car leaning towards performance, the less the car acts as designed for max drivability and economy. Domino effect. Change one thing, have to change 2 more to compensate for side effects. Like taking medications.
Street cars (old carbureted) make poor race cars and vice versa.