I have a 70 wagon that i'm doing and it appears to have all original paint on it. Do i leave it or take it down to bare metal?

if it looks good and you like it then keep it. If you don't like it and it's all worn then re-paint it, you will appretiate it much more. Try to keep its origonal color to keep its value but if you don't like that color then do as you please.

I have a 67 malibu and don't care for the origonal granada gold color so im thinking of painting it black or a blood red.

It depends on the condition of the paint. If the paint is solid, meaning that there is no cracking, chipping or peeling, then it should be safe to repaint. Even pinholes and tiny bubbles of rust can cause problems. In general terms, the sanding for paint prep will most likely knock most of the original paint off down to the factory primer.

The paint would be either a lacquer or enamel you would have to sand it down nicely and spray a sealer to prevent leaching and solvent absorption. Then a good layer of primer and topcoat. If there are minor "shopping cart" dings a little metal glaze can be used to fill minor dint's which DO NOT EXCEED 1/8". Anything past that depth would require the area to be stripped back to steel and filled with a good lightweight filler.

Hope it helps