Mohamed,
I cant understand why You want to use opamps.
There are two variants:
- scale is "legal for trade",i.e. must to be certified.
In this case there is one requirement - range of "zeroing"
at power-up cannot exceed 20% of full scale (600g ). At - 10grade Celsium and +40 grade( Celsium ) opamp's offset voltage (drift) maybe will exceed this requirement. This is in case when
opamp with low noise (but with high T_coefficient ) is used.
If "autozero" opamp is used, "new" added noise maybe will make things worst. I'm not sure,really.
In case of not-certified scale You can use low noise opamps,
but i can't predict will be or not successfully. It will be very hard to select right opamp (and to find).

On other hands - AD7730 has mode with 10mV full scale (with
smaller resolution), and signal is 5mV maximum. In addition there is DAC to remove initial offset ("dead load").

My advise:
1)do not use opamp, make AC exitation end enjoy.
2)even on prototype use SMD parts on analog side. AC bridge
can be "trough hole" prototyped, but input part must be SMD
(AD7730 too).
3)As i understand PCB is ready, but check carefully signal
paths AND ground path. AD7730 consumption (20..30 mA) can
introduce bad readings. Any power circuits (charger,LED) can
introduce bad readings via ground paths.
4)One little "secret" - Do not apply Exitation voltage
to Reference differential inputs - use resistive divider ,builded with 3 equivalent resistors in series- get signal from middle one.


Good luck,Mohamed!
Please, inform me about results.
Stefan Kanev
email:stefan63@abv.bg