Here's a photo of mine. This is on a BP347 with Holley Sniper2. In my opinion the extender is not required. But like Papa says, you will need the adapter. The sender that came with the gauges should have the adapter on it so it will fit right in. Mine came that way. If not it's just a standard brass pipe adapter that you can buy at Home Depot or Lowes. In my opinion you cap only the thermo switch wire in the front harness near the radiator. The other one you refer to is where you connect the black/yellow wire from the Holley I/O subharness. That's the output for the EFI to control the fan.

PM me if you don't have the Holley harness one page schematic and I can email it to you.

If you don't have a heater you can use the two manifold ports for your two water temps (EFI and Gauge). That way you can stay awy from using the one on the t-stat neck. That's less than ideal because it's on the downstream side of the stat so you won't see the correct temp on you gauge until the stat opens.

Pat

oil pressure sender lr.jpg