Refrigerant in a system needs to be at a certain level.
If you are converting to R134 then an expantion valve may be needed, but not always.

To much refrigerant and you will run high head pressures.
To low of a charge and the compressor short cycles.

I just did a friends 89 pickup which I believe was a R-12 system. No problems at all Filled it until the sight glass on the receiver was clear and called it good.

Systems with receivers are more forgiving than ones with out.

By the way you charge a system by the weight of the freon.
A system that takes 3 1/2 pounds will take 3 16oz cans and a half of a 12oz can.