The cam isn't holding you back and the exhaust and stock head should be plenty good for 4500 or more. Also the carb is big enough for that engine speed too.
Assuming that the ignition isn't breaking up at 3500 I would check:
1) that the carb is opening all the way,
2) that the distributor advance is operating properly and the timing is set at the right advance,
3) and that you aren't loosing fuel pressure for some reason at 3500 rpm; bad pump, plugged filter, kinked/flattened fuel line, plugged screen in the tank.
Tom
