There actually is a point to be made regarding replacing a rusted out trunk. The W114/15's are uni-body and as such almost ALL the body panels are structural.
Trunk, floorboards, bulk heads, etc. are designed to absorb impact should a collision occur.

The key to replace is whether you have surface rust only or has the metal's integrity been compromised.

A physical examination is needed.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now