How do you adjust the speedometer? I know that mine shows that I am travelling ~10-15 mph slower than I actually am.

Anytime you change tire diameter the speedometer will need to be recalibrated. You do that by replacing the drive and/or the driven speedometer gears in the transmission. When you change a transmission, if the speedo is correct in the old you should take the Drive and driven gears off the old one, assming its the same type of transmission i.e. 350 to 350 or 400 to 400 etc....

This should work for a 350 although I have seem better sites for this

http://www.tciauto.com/tech_info/speedo_gears.htm

This place has a good calculator also if you can catch them when their site is up.

http://www.bowtieoverdrives.com/

when you say driven gear, you mean the gear on the end of the speedo?
I am having the same problem and mister officer didn't believe me, so hopefully mister judge does!