Speedometer out of whack, switched Glide to TH350

You need a different speedo gear in the trans. What ever car that TH350 came from had a different rear ratio than your 65. Figure your rear ratio, then you should be able to figure out the gear you need. It is just a little coloured plastic gear.

This will help

Gear calculator:
http://www.tciauto.com/Products/Techinfo/speedo_gears.asp

You could peek at the PowerGlide gear as I believe they used the same color codes. Or at least tell the sales guy what the glide had in it to get a new TH350 gear set with the same number of teeth.

Big Dave