The boost gage on the tach and speedo are parts of two seperate and distinct gages in the cluster. You could sway out the tach to get a working boost gage, keep your old speedo, and put on the new face plate and you should be good to go.
I did the same upgrade a couple of months ago. If your old speedo is working fine, swapping the face is the easiest path. To do so, pull off the needle with a gentle tug and then release the three plastic legs holding the face to the unit. Put the new face on gently press the needle back in and match it to the 10mph mark and your done (note, I assumed the unit was already out of the dash and seperated from the cluster). If you decide that you want to use the new unit, rolling the odo numbers is a little more work and still requires that you take it apart (face off and pull out the odometer unit).
I just did this a couple of weeks ago, when I changed speedometers. It's pretty easy to either just swap elements (or roll the new odometer to the desired setting before the speedometer swap.
In either ase, you need to remove the odometer from the speedometer assembly. The little wheels ride on a metal bar, held at one end by a small clip with 2 small fingers which reach around and behind the back of the bracket. (This is hard to explain, but obvious when you are actually looking at the speedometer.) Gently pry the fingers back and remove the clip, then lift the "axel" (metal bar) up, and the odometer will come out in your hand. (Be careful not to bend or lose the little clip.)
To swap elements, just do the reverse.
To roll the odeometer, you need to put the center axel in the chuck of a cordless drill, and gently hold the high digit number barrel as you spin the shaft.
(You might be able to pull the wheels apart enough to allow you to slip the digits to something close.) Turn on the drill and either roll forward or backward the desired amount. (Note: When I asked this question earlier, several folks pointed out that using the "speedometer cable" input to roll an odometer is hopelessly slow, due to the gearing. When spinning the odometer shaft directly, a low speed cordless drill can turn roughly 1000 miles/minute. (try to run the drill at the slowest possible speed - the index mechanisms between number wheels are plastic.)
After the odometer reads the desired amount, you can just re-install it.