Welcome to the forum, and congrats on the car. That's an old girl right there.
You're right: too old for a ScanGauge.
I'm not even sure if it's fuel-injected (but I suspect not). If it IS, then you could install an MPGuino for real time MPG monitoring.
If it's not, well you'll have to go old school: vacuum gauge would help.
You could also try to finely calibrate your fuel tank's sending unit against a digital multimeter to get reasonably accurate trip-level fuel use (assuming you park in the same spot each time you check it).
You're right that driving style has the biggest impact, and changing that is free. But there are also a bunch of easy mods you could do, too.
Some light reading, in case you haven't seen these yet...
100+ Hypermiling Tips
65+ Efficiency Mods
How wide are those tires? They don't look stock!