You could do that... but its easier to do the speedcal and then you can easily change your calibration to any gear you want with the simple changing of a few switches. No tune/retune required. You can get the Dallas Speedcal here
Other places sell it too, use google.
The formula to get your corrected VSS output that you need for putting a 99-up OSS transmission in a 98-down VSS car is:
REAR tire revolutions per mile multiplied by your rear gear ratio multiplied by 12.
-You get your tire revolutions per mile here:
Tire Size Calculator - Discount Tire Direct
-Gears you should know, if you don't then find out.
-12 is the OSS pulse count of a 99-up trans.
You take the number you get and plug it into the Current Setup VSS pulses Per mile
block of the calculator here
Recalibrator & Speedcal Calculator
Do not put any commas in the number or it will not work.
The binary number you get in the extended range switch setting is the dipswitch setting you use. Not hard.