Wow. To be perfectly honest I didn't think the 80-92 tailgates would boltup to a 78/79. The fact that you have one is pretty damn cool and great info. I *know* the caps are different and don't interchange so I guess it's no surprise that the window openings don't line up.
I have to agree with Fred, about all you can do it measure the mounting points and see if they matchup. You should be able to get 78/79 tailgate glass from Jeff's but don't expect it to be cheap, it's probably more cost effective to buy yourself a parts truck so that after snagging the glass you can part-out the rest of it and recoup your expenses.
Please, let us know how you make out. Also, if possible *I'd* be very interested in seeing some photos of the 80-92 tailgate on there and just how well it lines up with everything. Like I said, most of us never even realized they were interchangable. Actually seeing the fitment would be great.