Yes the head will bolt but the 2.6 head has a squish pad and the 3.0 is an open hemispherical chamber. So a 3.0 head on a 2.6 would have a lot less compression. FWIW, there may be a few oddball assemblies out there but AFIK... no dice.

I wouldn't expect much gains from a head rebuild unless yours is in poor shape with excess deposits. Mercedes heads are pretty good castings out of the box but you may be able to get a little more out of it. Most of the work I did to my head was on the chamber side, the ports are hard to improve on.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff