Good luck keeping it 12v positive ground. Not sure why you want to do that.
As for the rest of the stuff, some nice to have, some not needed/ That fuel gauge converter you link? Too expensive and you only have one gauge that cares, the fuel level.
https://www.speedwaymotors.com/Speedway-Runtz-12-Volt-to-6-Volt-Voltage-Reducer,2374.html
Here's a clue any parts place that has VIntage in it's name, will charge you a lot for the privilege. Not near as much as MoParPro, but still.
Those 12v to 6v reducers?
https://us.rs-online.com/product/arcol/hs100-2r-f/71432076/
That solenoid? I used 1M1030, under $10 at rockauto.
https://www.rockauto.com/en/parts/wve,1M1030,starter+solenoid,4188