Anybody running 276 gears?
If you truly drive the car "a lot," overdrive is really the way to go.
I think you have to ask yourself: How much would it cost you to re-gear the diff? And how much would it cost to have an overdrive transmission installed? And what's the difference between those two numbers.
You will never make up the cost difference in fuel (I did the math, for me it would take about 85,000 km), but the driving experience is night and day with OD. The experience becomes more important when you're driving the car all the time, especially if you're cruising highways and country roads.
here's some quick maths:
518: $600 (CL/offer up)
comprehensive rebuild kit w/ shift improver & upgrades: 800
torque converter: 400~1200 (so let's split the baby and call it 800)
mount: 400
lock up/OD control kit: 150
yoke if you didn't get one: 50~100
shortening drive shaft: 150
trans cooler kit: 100
misc odds and ends such as fluid, biscuit mount, dipstick, wiring supplies, etx: 200
so you're in 3200~3500 easy money
this surmises that A: all the hard parts in the trans are good and you're not upgrading planetary gears, and B: you're doing all the work yourself, as well as C: you're okay with cutting up the car.
the OD option has all the pros of not spinning the motor to the moon at highway speed and getting *slightly* better fuel mileage which would only be important if you were long hauling it, tbh. but suffers all the cons of spending a grip of dough, a bunch of time and cutting up the car.
vs spending $500 for some gears and saying: ehhh, that's good enough.