yes, those look pretty short.
yes the alternator from the 300D will fit right in there, but you will need to swap pulleys over. the 240 belt is too big for either slot in the 300 pulley.
why don't you pull the regulator from the 300 alt, and compare lengths of brushes... and swap just that if it's good...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!