On my 300SD there are several threaded long bolts which are adjusters for the belts. Also there are bolts on the front and maybe one or two on the back of something (compressor, alternator? - do not remember - but when I installed a new AC many years back there was a bolt on the back).

Are you sure you have all the adjusters undone and you have removed the bolt in the back? At least the SD is designed so you no longer need to use a 2x4 to tighten the alternator. Much easier now than on my old '57 Chevy. Assume the 300D is designed the same way.

Not sure this helps. Are you sure your new belt is not shorter than the original?