Congrats on the engagement. It bodes well that your wife-to-be seems to understand and accept your interest in old Volvos.

I agree with the others that if the OD works - don't mess with it. Clean out the filter and give it fresh oil then give it a whirl. When OD's stop working, nine times out of ten it can be fixed by topping up oil, cleaning out the filter , freeing up the solenoid and making sure the electrics are sound.

If you have done any reading about fitting overdrives you will have come across plenty of how-to guides. One of the most important bits of advice is to make sure you have the lock out switch wired in so that it can only be engaged in 4th gear. It is possible to use OD in the lower gears but it is un-necessary and the higher torque doesn't make them last any longer. The main reason for the lockout is to prevent engagement in reverse. Running just a few yards with the OD engaged in reverse can mangle your OD big time.

I went through the exercise of upgrading to OD on my 63 wagon several years ago and made a tool to bench test it to make sure it worked before fitting it. I also sourced a shorter drive shaft from an 1800 so I didn't have to cut down the original.

If you feel inclined to bench test it - this link might be useful.

http://amazoniac.wikispaces.com/Bench+testing+an+overdrive

Best of luck with Volvo and wedding plans.