Speedometer help please!!

[collin70k/20]

The other day after we got my truck done we took it out on a test drive. And of course right out of the drive way my dad floors it and the speedometer went right to 100 mph so we thought that might of been the problem. So we turn around and we take off nice and easy at the stop sign and it still pegged at 100mph. The truck is a 1970 k/20 truck with a 350 engine and 400 trans. Any help would be greatly appreciated thanks Collin