you're going to have to find a shop that is able to install gears... they're relatively simple but they do require that you have 2 specialized gauges... one for setting the correct pinion depth and one for getting the correct drag... while you're at it you should get a gear installation kit complete with axle seals, new wheel bearings, and carrier bearings. Since you're only going to do this once in your lifetime, you might as well freshen up the whole rear end... its only about $100 for a kit. Also you'll need some 70w gear oil and friction modifier. If you bring all the goodies to the shop they wont charge you an arm and a leg.
As far as the speedo gear, you simply unbolt the speedo cable from the rear of the transmission and pop the old gear off and stick the new one on and bolt it back in with a little high temp RTV around the metal surface being careful not to get it in the tranny... also there is an o-ring that you could replace at that time so theres no leaks... autozone and kragen sell o-rings... just take the old one in...