The electronic to mechanical speedometer and the calibration problem can be resolved with the Abbott Cable-X box. It uses the electronic pick-up off of the side of the transmission and contains an electric motor that drives the speedo cable. I paid just under $300 for it and had a cable fabricated by a local shop for $28. The speed of the motor is controlled with dip switches. It comes with a chart for a starting point, and with a GPS, and some trial and error, you can dial in the speed exactly, no matter what the rear gear ratio or final transmission gear ratio.
I mounted the box under the driver's seat with Velcro. The speedo cable runs under the carpet in one of the grooves in the floorboard. There is already a hole in the crossmember under the seat that allows the cable to pass through. I connected the power wire to the back of the fuse box and ran that wire under the carpet in the corner of the floorboard. The wire for the pickup can run under the carpet and through the shifter hole and to the side of the transmission. If you do the transmission now, and the rear end later, you just flip a couple of dip switches to re-calibrate the speed. You can even compensate for tire diameter.
Unfortunately, this leaves you with no excuses for the local constabulary.