My 306's are always .030" overbore and a .010" polished crank
331 and 347 are the same animal with more clearencing and a 5.4" rod and matching crank.
Good question!
Unstable Performance
Most likely the green signal wire or the ground for the tach issue.
Idle could be many things start with IAC.
Turn signal Switch.
Enjoy
Unstable Performance