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For oil pressure, use a tee-fitting to tap into the stock oil pressure port in the block, near the oil filter. You may need an extension to give you room to screw in both the stock switch and the aftermarket sender.
For water temp, I just removed the stock sender on the driver side of the intake manifold (single wire sender) and installed the aftermarket gauge sender there.
To get a good oil temperature reading, you will need to mount the oil temperature sender in the side of the oil pan in a position where it will be submerged with oil even with the engine running.