You must lift the engine at least a bit before removing the heatshield(s) since the old mount will have collapsed onto it, pinching it in place.
Disconnect /watch fan shroud and throttle linkage as you lift engine. You have to lift it further than you think.
Do NOT let the main bolt x-thread when re-installing. Have a bar handy to tweak the engine side to side to align hole if neccessary.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)