Whmmm.... if I was a betting man, I'd bet that there's a wire in there somewhere that's not getting adequate connection between battery negative and ground.
Resistance in the high-current starter path will prevent sufficient current for the starter to pull in (Clunk, perhaps a RawrRawrrrrawr), but not enough to crank fast (ClackSkideeSkideeSkideee!)... and... if it happens to be that there's a marginal path between the battery's auxiliary ground AND the chassis, your ignition system is probably 'seeing' insufficient voltage to fire...
And something I see all-too-often in my daily work: Capillary action. Stranded wire is incredibly effective at capillary action, and as one would expect, copper loves nothing more than to conjoin with oxygen in some deep dark spot, to form fuzzy green powdery offspring, cleverly hidden from the world inside that insulation. Some might say they suspect I work on boats, too...
Feel your way down the major wires... bend 'em... if they feel a little 'crunchy'... replace them... or better yet... put a known-good jumper wire from battery to your C5 and see fi you get spark... I'll bet you do!