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In my experience, based on the information of several guys I trust, and seeing it done on mine, the best way to replace the windshield on a '79 F150 is to call the local auto-glass place and write a check.
My brand new glass was only $150 installed. My junkyard guy is a friend, but he wasn't going to have me out there breaking old, brittle junkyard glass learning how to remove it correctly -- and for what he would have charged to pull it himself, assuming I could get it home and installed w/o breaking it while learning how... it just wasn't worth it.
If you still want to do it, a 79 should have the two piece gasket first remove the beading in the middle of the gasket,pop the little tin corner brackets spray the gasket with WD-40 or similar lube take a small bladed screw driver and remome the bead.then go inside the cab remove the tin bracket/hold down from the ceiling behind the windsheild spay lube all around the gasket dont be shy, then start pushing out the glass starting in the corner ounce the old glass is out put the gasket on the repacement glass,take a piece of nylon cord into the area that goes to the cab put the glass in starting at the bottom with the cord on the inside pull the cord out as you push /fight the gasket in once it gets in all the way around push the beading back into the gasket again starting at the bottom then each side,then the top. Be ready to be frustrated the first time I did it ,it took me 3 hours 1/2 a pack of smokes and all the patiance I could muster. Some say I'm a glutten for punishment cause I done it 4 more times and broke one windsheild. Better yet pay somebody if you have the choice.
If you are doing a windshield get a new gasket. The old ones get hard and take a set so they leak. If you use a sealant you will be cursed until the end of time.