There usually isn't any problem in bleeding the cooling system on a 124 Spider.
You've already checked out the new thermostat in boiling water so hopefully that's ok.
There's a metal pipe inside the expansion tank outlet which can corrode badly and prevent coolant flow to or from the tank. With the radiator cap removed, coolant poured into the tank should run out the expansion tank pipe into the radiator neck. If this isn't the case, investigate further and correct.
Run the engine with the radiator cap removed and the heater valve open- a fast idle will speed things up.
As the engine warms, try giving the lower rad hose an occasional squeeze to displace any air pocket. Keep topping-up the radiator as necessary.
When the electric cooling fan kicks in, wait for it to stop, then top-up the rad for the final time via the expansion tank - this fills the line to the expansion tank. Refit the radiator cap. Top-up the expansion tank to the Max/Full mark (about 1/2 way on the tank) and refit the tank cap.
Don't remove the rad. cap unless the engine is cold. Coolant level checks are usually done at the expansion tank. ( this tank should keep the rad. topped-up and it's line to the rad. needs to be kept full).
AL.
