Thanks Hunter, yep, easy to pull!

Hmm . . that would be good to know - my sense is that I have rust klinging vs embedded in the surface and not sure why other than I know this car sat for some time with little to no fuel no doubt which created the situation. A thorough draining/cleaning followed up with a phosphoric acid clean might just do the trick. Wish I had room to get in there with a larger opening available for visual inspection and for easier mechanical cleaning -

If the tank sat with little fuel, water from condensation will create a lot of sediments algae/ grime over time. If the W115 is made the same way with the same material and coating as the 617 tanks and the coating is intact, you may luck out that it is not rusted. Take the tank out and soak it in a solvent that will not remove the tin/lead coating (not sure about phosphoric acid or any kind of acid). A really great solvent for algae and grime is this stuff from Dollar Tree store called LA Awesome Degreaser. Put some ball bearings in there and agitate and let it soak as long as necessary. Get a $15 USB borescope so you can look inside.

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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

I have had success cleaning fuel tanks on 1970's mototcycles with boric acid (concrete cleaner) and coating with RedKote tank liner. RedKote is a great product

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1982 300sd, new project car to restore

It doesn't matter if you know what you're doing, as long as you look like you know what you're doing.

My question was on materials and finish/ plating of the tank. Not where it's mounted.

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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

I thought concrete cleaner is Muriatic acid, which dissolves rust and steel.

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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

At this point, I think I am going to use phosphoric acid (which I have) or possible citric and thoroughly clean and may even use a curved tip on a pressure washer to blast inside with. I have some marine clean to use initially which is a great degreaser after tank is drained. Either way, a full day's work. If I discover I have embedded rust - I will use electrolysis so that the rust converts and then will fill tank full with diesel and monitor fuel filters, etc. carefully. Thank you everyone for the input!