First off, you have two different problems.

The speedometer is cable driven, geared off the output shaft of the transmission. To recalibrate, you have to replace the speedometer gear in the transmission. You're reading consistently 10% low, which means your rear axle ratio isn't a match for the speedometer gear.

To fix this the right way, you'd have to get the speedometer gear from a 3.07 car and install it in the transmission. But you can do a "poor man's recalibration" by repositioning the needle. Position the needle so that it reads correctly at 60mph...the error will increase proportionally above and below that speed. At 10 MPH, you'd be off by 6mph, but in the range of 50-70 where you need it most, you'll be close to correct. The trick to positioning the needle is to take the instrument out of the car. Use a small section of speedo cable chucked into an electric drill to spin up the speedometer, and note the reading. Whatever reading you have is the baseline. Remove the needle, and reposition it until your reading drops 10% from the baseline.

The tach is electronic. I'm not familiar with this year, but it's usually driven by a position sensor on the ring gear (I assume you installed the 240 flywheel?). Most of these gauges have a calibration mechanism, but I don't have one to examine.