I am planning to flush the XJ8 radiator, engine and heater.
The tech bulletin calls for installation of Adapter 100-022 into the radiator drain to back flush it. It is a simple adapter to allow connection of a hose. Any ideas on a substitute? Don't fancy paying $32.99 for one!

You could always pull the drain plug and take it to the local plumbers merchants and see if they can match the thread and put a few pieces together so you can attatch a hose.

Trouble is that it will be metric and everything here will be inches. Some sizes are interchangeable though.

Any idea if this adapter is the same for the X300 radiator?

The website describing the adapter states "Applications X400"
That doesn't tell me much.
The adapter 100-022 is described in TB 303-11 - V8 XJ/XK Coolant change Model range 97-99.