Sometimes this website can be helpful for decoding chevy engine numbers: http://www.nastyz28.com/chevy-engine-code-stampings.php
VIN code "H"
You must have an engine scanner to do that.
The engine codes for gasoline engines in the 1985 Chevy Celebrity: 2.5L 4 cylinder engine code R 2.8L V6 mpfi code w 2.8L V6 carburettor code x
You have a 1973 nova 350
Diagnostic trouble code
its Chevy 454 Big Block Engine
Engine cylinder # 5 - misfire detected
After repairing the cause of the code, you clear the code with a scan tool.
Trouble code P0306 means:Cylinder 6 misfire detected
EGR closed position performance.
1986