Hi,
The fuel sending units are the same whether you had standard or ralley gauge clusters. The gauge and/or sending unit is out of calibration. You should have both units re-calibrated together to insure an accurate reading.
Pretty much all GM gauges after 65 were air core technology and used a 0-90 ohm scale. Most new senders you can get now are junk. I have had good luck with the ones sold by Lectric Limited.
Pontiac had two different oil senders in the 60's one for a 60lb and one for an 80lb gauge. For 69, you'll have an 80 lb gauge so make sure you have the right sender.
It would be a good idea to have the gauges checked before buttoning up the dash.
I'd be happy to help with any questions/problems you might have.
Thanks
Mike Mancini
www.instrument-specialties.com