Visual Differences
The easiest way to differentiate between  the two models would be the wheels, which on the P5B are chromed Rostyles.  
Other points to note would be side indicators, no hole in the front grille for the starting handle, rubber strips to the over-riders and recessed fog lamps to the front wings - all found on the P5B.

The 3 litre had automatic and manual gearboxes, the V8 only  came with an three speed automatic.