After a bit of research, I found that I could buy two gallons of QuietCar for about the same price as Dynamat impostors. QuietCar is a liquid sound deadener that is actually lighter than the Dynamat-esque, stick-on mats. After a shameful, argon-less welding job to replace an inexplicably mangled seat mounting bracket, Jay and I began brushing on the first coat of QuietCar. We'll furnish a second coat later this week, but the fisrt is going on nice and thick.
The stuff is kind of goopy, but it's really forgiving.
Next step: Headliner and carpet from Esty.