Since you're still not looking at a major build I'd match the crank to the electronics being unless you have a specific reason to do otherwise or one of the cranks is in poor shape. The 7M crank would be my preference out of the two as a starting point for a big build though but it would end up somewhere in between a 7M and a 6M as far as weight and form.
The rivet shows there's a MLS HG installed. The stamping was put there by a shop who worked on it... I'm really only used to seeing anything more than a label with serial number on the timing cover unless a very significant amount of work has been done to the engine and or head. Any similar markings on the block?
The scribble on the side is a positive... thus you know it was actually something cut into the mold. It's a mold identifier, you find those on all heads. Sometimes there's just one or two numbers and sometimes there's a good deal more. It usually is information used only at the plant and just specifies which casting revision it is and specifically which mold. IIRC, there are 27+ known different castings for the 7M head... I'm not certain about the number right now, I'd have to sleep on it.
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