I would recommend using a timing light with a digital tachometer readout to verify how far the tach is off. Tachs can be rebuilt, or contact your Four Winns dealer for a replacement. I did that on my 268 for about $130 a year ago. The tachometer has an adjustment screw on the back to set the correct number of engine cylinders. Make sure this is set correctly.
The trim gauge rarely goes bad, it is usually the sensor on the right side of the outdrive.
I would check the tube for the speedometer for blockage. The gauges in most of 248s and 268s are attached to a decorative plastic simulated woodgrain panel held in place by 8 screws. Remove the screws and the panel with all the gauges can be pulled back out of place and laid in the dash to rotate and work on. There should be plenty of length in the wiring harness.