I think the ribs are for strengthening the tank against implosion when under vacuum. What did you replace yours with? If I were to replace mine, I'd get one (from an SDL?) with a level float switch and wire up an audible alarm.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked