[evintho]

The reason your gauge goes to full on an empty tank is because................
A) The resistance on your '67 sending unit is different than the calibration on your '88 gauge, or...........
B) You have a bad ground.

First find out what the resistance is on your '67 unit. The calibration on your '88 gauge reads 33 ohms as empty and 140 ohm as full.
Next, run a seperate ground wire from just behind the connector that comes off your tank to a dedicated, good ground. That will confirm a good ground.
The problem is probably A.