Does the engine still have the original Vac pump? The cover on the old style was crimped on while new ones have four torx head bolts on it. Bad vac pumps have probably destroyed more 603's than the cracked #14 heads I think! At least a cracked head is not in itself a complete disaster unless you let it go too far maybe.
The first thing to remove is the vac pump. If its ok, no little bits and pieces falling all over the ground, then I'd start looking at the timing gear, and that means the both manifolds and then the head can come off (be aware of the proper sequence for loosening head bolts!)
I don't envy you, I just tore down the top end of the engine in my '87 300D last Fall. I did not have to remove the front timing cover, as there were no problems with it turning over (it had a cracked head!
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I saw a bad vac pump and what it did! it locked the engine up AFTER the owner's daughter drove it several (many?) miles while it was making a loud clicking noise. No squealing, just clicking then Kraby Kaputskie, the ball bearings jammed the IP timing ramp and the the vac pump cracked up in parts and the engine locked up because the timing chain was held frozen. Your sounds like it may be a spun bearing though hard to tell at this point.