Will this replace a 40/5 MFD dual capacitor?

It can if the fan circuit tolerates 6 MFD instead of 5. Match the compressor section exactly at 40 MFD and keep the fan value within the motor's rating; when in doubt, use the value printed on the old capacitor.

Does the 440/370V rating work on a 370V system?

Yes. A 440V capacitor is rated for both 370V and 440V circuits, so it is a valid upgrade. The voltage rating must never be lower than the original.

What are the two values for?

The 40 MFD section runs the compressor or main motor winding. The 6 MFD section runs the condenser fan motor. Each terminal pair is wired to its own circuit.

How do I know which terminal is which?

The terminals are grouped by capacitance value, usually marked on the case or label. Check the markings before wiring and photograph the old capacitor's connections first.

What is the physical size?

The round aluminum case is 4 11/32 inches tall overall. Check the diameter and terminal spacing against the mounting bracket and the old capacitor before ordering.

Is it safe to install myself?

Only if you know how to discharge a capacitor and work safely around line voltage. Capacitors hold a charge after power is off. If you are unsure, have a technician install it.