core for inverter transformer

I want to make a inverter transformer of 400w. in my local store i
found many kind cores, all from old transformer mainly from ship
breakings. some are heavy others not so heavy. some have black paints
on it, some are gray, silver golden etc. I want to know which type is
better for inverter transformer.

and the transformers primary 12-0-12v should have bifilar winding.
should i wind the secondary 220v loop over the primary? or should i
wind them side by side.

thanks

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