Here is how I tested my '78.
1. Get a bottle of leak test solution or use very soapy water to help find the leaks.
2. Remove brake booster line and connect a hose to supply compressed air regulated at 3-8 psi going into to airbox.
3. Secure latex gloves covering the air box intake for the filter, another one on the exhaust tailpipe(s) and one on the oil fil cap.
4. Turn on the air and wet the injector holes, airbox seams, airbox rubber connectors, all rubber and vacuum hoses etc. with the leak detector (bubble) solution. Watch for bubbles to be constantly generated where the air is leaking
Here is a pic of an airbox that was leaking at the seams during a bench test after using the same method described above.
http://forums.pelicanparts.com/6692499-post64.html