On my 83 300DT, both lines are the same diameter. It is a metric size and from memory, I measured the metal lines at 15 mm dia. A 5/8 hose is a bit on the loose side so is not ideal but with 2 good clamps it should be ok IMO. I'd use one worrm type clamp and the other a constant pressure spring type clamp which will self adjust as the hose ages.

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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