I've thought about it, but never did it. As you probably know the 2.6 and 3.0 blocks are different. The stroke of both is the same, but the 3.0 has larger and siamesed bores, unlike, for example, a small block Chevy, which went from 327 CID to 350 with a simple increase in stroke. DB never seemed to do anything the easy way back then.

I believe the heads are also different with the 3.0 head having larger ports and valves. I don't think the 3.0 inlet manifold or air cleaner will work due to less vertical clearance under the W201 hood.

It sounds like you have both engines available for inspection, so I think all you need to do is verify that the 3.0 block has all the same threaded bores to mount all the 2.6 peripherals. The 2.6 EZL and ECU should work, especially if both are the same model year. There may have been changes in some years in the number of pins, but if only internal calibration they should be interchangeable.

Using all the 2.6 components other than the block won't get you much more top end power to the the 2.6s more restrictive inlet manifold, head, and exhaust system, but it should make noticeably more low and mid range power, which is more useful for normal driving.

Duke