Thank you that is good information just what I was looking for .When replacing the original Pontiac OHC-6 with a Pontiac 350, there are a few issues you will have. And as your FB is a 1967 model you will have 1 more thing to consider, the 67 Firebirds with the OHC-6 used a throttle rod not a cable that attaches the gas pedal to the carb. So it will have to be replaced with a cable. I do not know how it attaches to the gas pedal compared to what you have. The other issues are as follows.
1 - Fuel Line placement, the OHC-6 cars have the fuel pump mounted on the passenger side - the V-8 cars have it on the drivers side.
2 - Battery placement - See above same applies as it does to the starter
3 - Motor Mounts and subframe plates are different
4 - Different Radiator may be needed, but as you are going to a manual trans it simplifies it as you do not need a transmission cooler (that your stock radiator would not have if you still have the 2-speed ST-300 Transmission, they were air cooled)
5- Transmission crossmember for the manual trans cars is different
6 - Fuel Line Size - if the OHC-6 is a 1bbl model the fuel line will be to small to feed a performance V8's needs.
Thats off the top of my head, I am sure there are a few things I am forgetting.