351c wont start, help

[1969_Mach1]

I think you need to fix the slow cranking problem before going much further.  Could be the ignition timing is too far advanced.  Could be a bad ground or positive cable.  Could be a bad starter.  I still don't understand the ground setup you have.  There should be a cable from that location Mustangmike circled to the engine block.  And another cable from the engine block to the battery.  Look on the rt front side of the block below the alternator.  If it's like all other small block Fords there will be a threaded hole in that area  That is typically where the ground cables attach to the block.