Tim.........one of the reasons to convert to 12V is the FASTER spinning of the 6V-starter motor on 12V for eazier starting of worn low compression engine. The 6V-starter motor does just fine on intermittent 12V starter duty. There is NO problem, connecting yer ignition ON switch to the right side of the 3-terminal starter solenoid as you described. Iff'n you connected to the left side of yer 3-terminal starter solenoid, yer engine would only run while pushing the BIG thumb starter button by the tranny shifter.
Remember, yer 30-0-30 ammeter was originally wired for positive (+) ground and will read BASSACKWARD with negative (-) ground that yer 12V-alternator demands. Iff'n still the original "loop" ammeter, just thread the ammeter wire thru the loop the OTHER direction, otherwize swap leads on back of 2-terminal ammeter. Surprizingly enuff, ammeter don't care, positive (+) ...or... negative (-) ground and even more amazing don't care iff'n 6V or 12V.
HOWSOMEVERS: both the original 6V-squarecan coil and the modern 12V-squarecan coil use the same "infamous ballast resistor".......NO ARGUE!!! ........Dell, a 12V advocate for the right reason, who knows 8-ways to convert to 12V and they all work the first time