To calculate the financial return on an overdrive conversion, set a price for a gallon of gas. In this case, I will use price of $4 a gallon. I hope this is a real horrible estimate of future gas prices. I am using 10,000 miles a year for driving. I am using 15 mpg before adding the overdrive.

Before:
10000 miles / 15 mpg = 666.66 gallons of gas.

Cost = 666.66 X $4 = $2,666.66 per year

Add overdrive and you get 25% better gas mileage or 18.75 mpg

After:
10000 / 18.75 mpg = 533.33 gallons of gas.

Cost = 533.33 X $4 = $2133.33

Savings $533.32

Cost of overdrive = $2,695.00 (Barracuda 904 price)

Pay back 2695.00 / 533.32 = 5.05 years.

If you drive less, it will take longer (5000 miles, 10 years). If gas is cheaper, it will take longer ($3 gas, 6.73 years). And both, 13 years.
You will have to figure this out for your own needs but just follow the formula above and you can see for yourself.
It makes no sense to put one on a trailer queen.
The hard to calculate advantage is less wear and tear on your engine and transmission. Since they are spinning at 25% less RPM (at cruising speed), that savings is more than negligible.
The fun factor is impossible to calculate but it is enough that I probably will do the conversion myself, but not immediately. My rebuild is first priority, the overdrive will be a year or so out.
Puting 25% less money into Exxon's pocket: priceless.

Craig