Is your engine Series II or Series III? With a 3800 SC Series III engine, computer from a 2006 GTP, and VDO instrumentation, there are a couple of potential issues that I found. The computer tach signal will not drive the VDO instrument. The only two solutions considered to date are either a new ignition systems with a tach output or rebuilding the alternator to include a tach signal from that component. No ignition systems with a tach output that work on 3800 SCs was located so I used a pulse modification on the old alternator to drive the tach.
The other two issues discovered were resolved by a tee in the oil pressure line for the VDO sensor and a new tap for the VDO temperature instrumentation. It would be a much better solution if the two oil and water pressure measurement points were done prior to engine installation and integrated into the harness. I have never experienced the instrumentation pegging problem that you referenced but the oil pressure is around 70-80 psig on the Series III engine so your sensor must include the correct range. To calibrate my VDO speedometer I used a GPS and the initial set was within two miles per hour. Then you can use the VDO incremental trim process to match the GPS and speedometer. No additional buffer was needed for the speedometer.
[This message has been edited by hnthomps (edited 04-19-2014).]