I would not go changing the Drive Gear just yet. You might be able to get away with changing the speedometer gear only and not the drive gear. They had alot of different combinations even in the same year of car. For example, you might have a pink speedometer gear with 24 teethe and a drive gear with 9 teeth, or a green speedo gear with 42 teeth and a drive gear with 18 teeth (and yes, I am just making up those numbers because I do not know all of them off the top of my head). So, you might just do alot of testing with the speedo gears before you go changing the drive gear at all. The gears them selves are only about $8 to $12 each so it might take a little bit of money to get it right, but you can also retuirn the gears that do not work for you.
Changing the speedo gear is eash on the Turbo 350 tranny. Follow the speedo cable down to the drivers side of the transmission and un screw iot from the case. Once that is off of the unit, there is one bolt that holds a clip which holds the unit in place. Remove that bolt and clip and you can pull the unit and speedo gear out of the transmission case. You will loose a little bit of tranny fluid (and I do mean a little, like 20 to 30 drops or so). You can then swap out the speedo gear, and re-install it back into the case with the clip and bolt. Then hook back up your speedo vcable and take it for a test run. I found that it is easier to have a car either follow you or lead you to pace the car for an accurate reading. I say that because I have never really used a GPS to calculate my speed. And if that gear does not work right, return it, get the next one and try again. Maybe over this whole process you might have to add 1/4 quart of tranny fluid but that is it. Each change out takes about 15 minutes to do. If that does not work, then yes, you can remove the four bolts that hold in your transmission tail housing, remove it fron the tranny mount and replace the drive gear as well if you can find someone who knows how to decipher the right combination. Like I said before, even in 1978 with the Olds 403 and 350 tranny, there was like 6 or so combinations that they used in the same year.