Heads are not the same
I have two heads in front of me, and one says 2.6, the other 3.0. There are certainly minor differences; even though rockers and camshaft are identical, valves are different.
The 2.8 is a (later) M104 engine - completely different. No way it could fit!!! Perhaps a 3.2 head would fit on a 2.8 but I wouldn't try it the other way around.
My two bits
1939 170 - till I was 3, still turns heads
1989 300TE - daily driver has 263k
1987 300E - purrs happily with 120k
1983 300SD - keeps my friend happy