Here are some pics of my rustbucket with some insights into what I'm seeing, and the reason I think the '77-'78 tanks maybe aren't the best option (unless you're good with a grinder and a welder

).
EXHIBIT A: Passenger side mounting points for a 1976 Aspen wagon. 1977 tank shown (no top valve hole).
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This tank is lifted up to the point that it's making physical contact with mounts. The mounting points (yellow) are inside and directly above the tank itself (red edges), which makes it seemingly impossible to get a strap around and actually reach the bolt. The original strap I have doesn't come anywhere close to reaching around this.
EXHIBIT B: Driver side mounting points.
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This is a composite image I made, so perspective and distance is a bit weird. These mounts (yellow) are directly against the floor of the car, not on the bottom of the spare tire box like the other side and like the pics posted earlier. The tank is pressing against the passenger side mounts and cannot be lifted any higher. I could possibly build a spacer to provide pressure against the tank, and the extra distance would provide access to the bolt despite the depth of the tank.
EXHIBIT C: filler hole distance
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The strap mount is highlighted in yellow just for reference. The red is the location of the filler hole. There's a significant misalignment. I think I would have to cut off the filler pipe before the bend and run a rubber tank hose in it's place. I'm actually not sure what's supposed to bridge the gap here, since the tank didn't come with any other parts and clearly there's nothing existing on my end to seal the system.
Anyway, just wanted to add these for anyone else that is digging around on the internet and looking for solutions.
EDIT: Oh here's one more shot of the overall spare box for the '76 wagon.
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