$300 for a fluid change!? I think you could buy a used transmission and get it intalled for not much more.

When I bought my 1990 740 wagon (180k) the transmission fluid was black like diesel crankcase oil, but there were no operational problems. I added AutoRx, drove the appropriate number of miles and then started a gradual fluid change.
I drained the old fluid 2 quarts at a time and replaced it with SuperTech Dexron III from WalMart. I repeated this process 11 times over 8,000 miles and replaced 98% of the fluid. I used 24 quarts and as I recall the SuperTech cost $35.00. I've driven 35,000 miles 100% issue free since. Doing the "flush" as described in the FAQS would be faster and cheaper.

My own thinking is that "transmission fluid change broke my transmission" is a myth. The old transmission was clapped out, ready to fail and the fluid change = failure is a pure coincidence. I suppose the new fluid could disturb the seals and start leaks, but topping the transmission fluid will be cheap until you can do a real fix.

Finally, I believe that you have the AW70/72 transmission and like so many older Volvo drivetrain parts they'll stand incredible abuse before failure. If this was my 740 I wouldn't hesitate to change the fluid. Forget the snake oil unless you use AutoRx before a change.