VSS is always based off of the final drive, it's never off of the engine. The Vehicle Speed Sensor is for the Speed gauge, not the RPM gauge, so if you are looking to use this for RPM sensing, it's not going to work.
If you actually want a RPM gauge, guaranteed there is an RPM reading going to the ECU, you just have to look for it. There are several aftermarket tachometers available from places like summitracing.com or restoration retailers. It's easiest just to get one of these. They will have ways to select the division factor for different #'s of cylinders.
There's some really simple cheap ones that are just LED line displays, if you don't want to get too fancy.