Is the heads stock or ported?
If stock I would guess 375-400 HP.
I know the Stock guy's with 340's make more power, but I don't know if the ARE Stock.
As long as they don't pour the heads for cc's I don't trust them
I built a 366 for a friend with a TIGHT budget(he is poor as he!!)
I've used std J heads with 2.02 intake valves and no porting, a Edelbrock RPM, Lunati voodo cam, and a Thermoquad.
Car is a original 62 4dr Dart with 8 3/4 3.23 and pushbutton 727 and Streetfighter converter.
Best et 13.02@104mph

Don't know the HP but we are happy as it's a NO money budget.