I'm trying to visualize what you are describing...;squint:
Do you feel like you are getting any resistance from either the inside or outside door handle? (resistance other than the spring in the handle itself)
Do you get any resistance or movement on the manual lock?
There really shouldn't be anything keeping the door panel from coming off if you've removed the screws. It might be really sticky/stuck with the tar-like coating that you might have around the edge of the panel where it mates to the door frame. Off the top of my head, I think there's 6 screws that need to come out. Two in the handle, one in the handle cup. And then I think two at the bottom and one towards the top-front??? (It's been a long time since I've had my doors off and I don't know if the pattern is different on a '77)
Post addition 1: How will you get to the door hinges if you can't get the door open???
Post addition 2: I can copy pages out of the shop manual and email them to you if you'd like. It gives a breakdown of the door, lock, and hinges.