84*86 truck 318
The numbers are on the side of the block. Look for a standard 6 digit number - displacement - single digit
there's also a casting date on the block.
You'll have to change the oil pan for it to fit in a barracuda. No big deal other than that. 727 transmissions will have a bump out on the passenger side pan rail where a 904/998/999 will be straight.
One other thing you'll likely have to do is change to a non-feedback carb and a non lean burn or electronic spark control ignition system. Otherwise the engine will work great.