keep in mind, the load that inverter will be putting on the 6v side will likely be larger than running more incandescent lamps.
the 6v led's will likely be the best bet. 2 in front, one in rear.. etc.
As for the op's charge situation.. I'd replace the ammeter... if you are using 30-35w lamps.. you should still have generator capacity left for charge with 2 lamps on.. 2 35w lamps should, at most run 12a... as long as you are not ideling, you have 8a of nominal capacity for charge and ignition left over. and with 3 lamps on you would be right at break even with ignition.
The charge amps will taper down as charge comes up.. usually to only about 2a.