cam duration

In a 340, The 340 cam should easily be tuneable. IIRC that was a 268 cam on 114LDa
That puts the ICA at 64* and that sets the DCR at 7.57 and the pressure to 148.
that should be easy to work with.
The first thing I would do is to check your cylinder pressure.Then I would check the timing and rate of advance.Your dizzy may be in need of an overhaul.