I was checking the A/C in the '77 Saturday after replacing the vacum control module in the

when you say the clutch is energized I assume you are giving the clutch a seperate power and ground and it works...one other check I do is jump the low pressure switch with the car running and the ac on...a  paper clip works fine...this checks the entire electrical system in and out...if it doesn't come on then you have an electrical problem..if it does you have a low charge...that was my next question..does it still have R-12 in it or have you swapped it over to R-134 yet?  really it comes down to can you find R-12 in your area and for how much and also how fast it leaks out if that is your issue..134 swap kits are cheap and easy...you may have to get a fitting for the back of the compressor to fit a 134 adapter but I found them at NAPA for a couple of bucks if thats the route you want to go..also check your clutch fan if you still have one..make sure it works like it should..on and off with temp..

I'd also start by checking the freon to see if it is low or not. Also check the fuses for the compressor-A/C system. Jump the low pressure switch, as Rich says.
Don't run the compressor for a long time if it is low on refrigerant...