Hello there Rev. The starter will be alright and turn the right way, but your heater motor may not. If this is a Ford setup, you might as well change the solenoid to the twelve volt one then one of the little posts on it will give off 12 V to go directly to the coil while you're cranking with the starter.You can't use the original old starter button if you use a 12V solenoid, because the old button grounds to the dash, and you want to send electricity out to the new solenoid. If you have a start position on your key switch you're laughing, otherwise you'll need a two prong starter button; one prong to accept electricity from a live source and one to send electricity to the solenoid.
You already know about changing all of your lights.
Your manual gauges will be alright including the ampere one, although it might read backwards,[reversing the polarity on the back of the gauge should work]. The straight electric 6V gauges won't handle 12Vs successfully. I have replaced some 6Vs with a 12V gauge and put the old 6V face on the new gauge and then reduced the voltage a lot to make them work. Good luck.