I would definitely loosen the driver's side mount. When I replaced the mounts on my '87, I replaced both while the engine was supported and then started the bolts into the mount. Afterwards, I lowered the engine. I don't recall having any clearance issues. Good luck.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold