I wrote to swap-section but no-one noticed it, so i wrote to here too. Im about to change the original mp/h gauge to km/h (from 71 240z) and i wonder if anyone has experience to share? Also im having a "new" 5 speed gearbox and i've heard that you must change pinion gear to get the correct reading to speedometer, what would be the correct gear?

It all depends what the diff ratio was of the donor vehicle. The correct gear is in relation to your tyre size and diff ratio.

Try what you have and check against another vehicle. It may be OK.

Tranny is from stock 77 as far as i know so the diffy is 3:545 right? Tires are 195/65/14