I did this on a '79 that I used to own. It had A/C, so removing the heater/AC box is a PAIN. (plus I never thought of it!) I took off the drivers side fender,and one hood hinge and put a shop vac on the water drain hole, then blew compressed air in the cowl and used brushes in thru the hinge hole. the reason I took off the fender was that I had a mysterious rain leak on the driver's side that I could not find. After all that, it turned out that the rain was splashing in thru the door because there was no tarpaper shield behind the door panel!