Ok, I might be able to offer a little info here. I'm finishing up the last details of putting a TPI engine into an '82 300SD. I kept the rear sump pan, but had to cut and reweld it a lot, and removed the stock Mercedes steering damper as well. I've gotten conflicting advice on whether the steering damper is needed, and haven't had a chance to drive it at highway speeds yet.
As far as the GTO pan goes, it might work, I've looked at the idea of putting a front sump pan on mine, but you might have to modify the cross member, I don't know how different the 300E is from mine. I didn't have to modify the cross member on mine, but wish I would have, pan clearance is tighter than I'd like.
As for the trans tunnel, I had to cut and widen it 2" on the passenger side, and it's still pretty tight on both sides.
Everything about the swap has been a real challenge, from the trans tunnel, to the shift linkage, to the Benz electronic speedo, to the air intake routing, to the exhaust routing, yada yada yada. I designed and built everything on mine, and it's been a long and technically demanding project. Mercedes electrical systems are pretty damn complex, and I don't consider a swap done until every gauge and accessory works properly.
Like GNX7 says, not an easy swap at all, although my initial test drives are very encouraging, looks like it'll be a fun, nice driving car. Kinda like a luxury muscle car.
Unless you have access to plenty of power tools (I used my bridgeport mill, lathe, mig welder, and 2-post lift a bunch!) and pretty darn strong fabricating skills, I'd advise not doing it. A typical LS/RX7 swap is a LOT more straightforward.
