Have you thought about swapping the rear pumpkin for a 2-speed unit? That's what I did on my Fire truck. It had a single speed 7.17:1 and I found a 6.14/8.36:1 2-speed that bolted right in. The total cost for the pumpkin and shift switch was $250 and to have my driveshaft modified to fit the larger 2-speed yoke was about $80. My top speed went from about 53mph to 63mph! My truck has an Eaton rear...don't know what type you have.
Swapping in a complete 2-speed rear (drum to drum) might be cheaper than buying an Overdrive unit.
Mike
