Replace the radiator... don't gamble on that neck... it will crack again.
The tranny cooling line is easy and simple...very little oil will drain out.

For now. Many people have tried to repair this to no avail. The price for
failure here is high. One guy did clean it scrupulously, apply JB weld
then wrapped the whole neck in thin metal and that's worked.

But most people replace the rad. They're not too bad mail-order.

I've never had a 300E but in 108's/126's there's an oil cooler on the
side of the rad. It's a separate unit and have never heard of one failing.

Guenter,

If I were going to attempt a repair, I think I would find a metal sleeve
that I could insert into the existing neck segment, then I would use a
silicone rubber adhesive to seal it neck back together again, with the piece
that broke off.

You may also put a sleeve on the outside of the repair to give added
strength.

Replacement may be a better option.  But if I understand the cause of these
failures often it is the type of anti-freeze that causes the plastic to
become brittle.

If your car is equipped with an oxygen sensor be sure to select a grade of
silicone that is listed as safe for these as it is my understanding that it
does not take much vapor from the curing rubber to kill off the ox sensor.

I own an '89 300E, my radiator broke the same way.  Unless you can
create another lip on the break area, replacement is best.  I replaced
mine in about 15 minutes.  It cost 135 bucks online for a new radiator.