Converting from idiot light cluster to gauges

[CanadianPoncho]

I'm in the midst of converting the standard idiot light cluster on my 69 Pontiac Laurentian (Catalina body on Impala chassis) to a "rallye cluster" which has oil, temp and amp gauges from a 69 U.S Bonneville. I converted the pin locations on the harness plug to match the Bonneville. That went smooth. I plugged in the new cluster and have the following problems:
The gas gauge only reads 1/4 even though there is half in the tank.
Does anyone have a GM parts book who could look up the PN of the sending unit to see if the rallye cluster cars got a different one? Not sure if this makes a difference, but I did an Ohm reading of the new gauge vs my old gauge and the new one has 20 Ohms higher resistance.

Also, my oil pressure reading is only about 12lbs cold. I swapped the idiot light sender with a Chevy gauge unit (new). Did Pontiac use different sending units for their oil pressure gauges?
I also swapped the idiot light temp sender with one from a 68 Nova and the temp guage reads full hot as soon as I turn the key on. I assumed that GM would have standard oil and temp sending units across the board but now it seems I am wrong.
I appreciate any help! It was a real pain pulling the cluster and I'd hate to have to put the old one back in. Here's a pic of the Bonneville cluster. They are pretty rare:


Todd