Had to do a search for the term "STV valve", never heard it before. In the 67-72 trucks there is a POA valve. Seems to be similar. Here is a post on adjusting a POA
http://67-72chevytrucks.com/vboard/s...&highlight=POA may help?
The other valve in the system is the TXV. Do not recommend adjusting the TXV. Which one are you referring to?