[bruceman1968]

You have a 1969 307. Your casting number 3956632 is for a 1969 to 1973 passenger car Chevelle Camaro Nova or Truck. I'm sure that you have a 5 stamped in your block, but when the 5 was stamped it was intended to represent a S. They did that all the time at the engine, transmission and rear end assembly plants. I's used as 1's are very common. lots of cases documented where letters were used as numbers or numbers used as letters. Sometimes a worker would misread a stamp and put it back in the wrong spot or a commonly used stamp would wear out and a similar looking letter or number stamp would be subsisted.
Your application code of U5 is meant to be US.
In 1969 US decodes to a 1969, 307cid, 200hp, manual trans, 2bbl carb, Truck motor.
If you want you can check your date code(opposite flange from your casting number) just to be sure.
EDIT: U5 followed by a letter was used from 1986 to 1990 for 350 truck motors. But your casting number eliminates that as a option.