My goals are durability and economy. This is not a hot rod, but if some machine work will improve the oiling system, economy, and the lifespan of the engine I should probably do it. Question is what is the best combo of work, parts, and cost.

You're saying that the double-valve springs would be good if I was going to change out my cam, lifters, and pushrods for a high-performance engine, but are really unnecessary if I have hardened seats put in and kept driving the car as I normally would?

That makes sense. Would there be any fuel economy advantage to "porting" or the three-angle grind as Craig suggests? Or at least will there not be a cost to economy if do some of that work and I drive the car normally?

After adding up the cost of all those goodies (without the machine shop work), I'd like to do what I need to keep my engine running tip-top without going overboard.

I'd like to save the cool stuff for the B20E in my 67 wagon. At this point I should really throw my cash at solving my economy problem vis-a-vi the carbs or getting a new electronic ignition unit.