dumb question about 327

I have been looking for a 327 for my 67 el camino,a guy is selling one and says
it's a high performace, what do I look for?, (on double hump heads)it has M/T
valve covers on ,a 2 barl. carb,and too dirty for me to see numbers, I also noticed ..no holes on valve covers,back breather on block was pluged with hose and a bolt, do this engines have to have more than just the oil fill cap
to breath?:confused: just for the record it's on a 68 chevy pickup but I don't
know if it's orginal, thanks for any input.....

Start clening the motor up to get a looksee at the numbers, you are looking for casting numbers and date codes