I'm sure you could retrofit one for the other if you were clever. But it would certainly involve more work hunting down or fabbing up your own early style mounts, some re-routing of fuel lines and electricals and whatnot, making a T-notch for the strap (assuming it's not in the '77 frame already that is) and likely a few other things.
In other words, anything is doable. It just depends on how much work you actually want to go to and how much your time is worth. At some point of diminishing returns, it's probably more cost effective for some of us to simply buy a large capacity tank.
But if you can find the early supports and tank, fit them to the frame, you could be good to go
I think it's been tried before. Don't remember the results though.
If it was me I'd hold out for the larger '77 stock tank and correct skid plate/mount, or if I didn't have two friendly members offering me one, get an aftermarket one instead.
Paul