suggestions for best way to change from 6 to 12 volt system

More trouble than it's worth, in my opinion. There's nothing wrong with a 6 volt system in good working condition. If you want stereo get a boombox.

Don't.

Why?

Leave it alone, like God and Louis Chevrolet intended it to be!

A Car or Truck? If it is a 49 to 54 car National Chevy Association sells a complete changeover kit with instructions go to:
http://www.nationalchevyassoc.com

You can get a jump start from a 12 volt battery with no problem.Just be sure no lights are on............... If all grounds are good 6 volt lights are as bright as 12volts.(using equal candle power bulbs).I see a lot of volt 12 volt cars with yellow dim lights.To get good grounds install shake proof or star washers under terminal nuts or screws to insure good connections.Make sure all wiring is in good condition and switches have good internal contacts (or replace switches).It is no more difficult to make the 6 volt work as designed than to change to 12 volt

I never thought I needed a tachometer for any of my older vehicles.

The only reason I would consider going to 12 V from 6v would be if I was going strictly for a cruiseing car and wanted to put in electronic (breakerless) ignition or more important from my point of view would be for Air Conditioning, some folks do live in places other than The East and West coast areas where it can be un-bearable from mid spring till mid fall without A/C and I haven't found a practial way to adapt the 12 v compressor clutch and the A/C blower to six volts.
The easy way is to drive a 55 or later car, but I will stay with the 28 and 51 and 53s for now and do without A/C rather than change from a 6v system that is perfectly suited for these old cars...., at least till I can buy the 62 Sting ray coupe or dual top beauty that Will be my next investment, and it will be all power and air and automatic and will have radial tires and tinted glass, Yeah!!