If I were you I would get a really big Allen key to fit into the shim housing "the Brass insert" in which you put the injectors into. Hopefully the old rubber o ring is enough to hold the injector head long enough for you to undo the shim! If not and it falls into the block you are in the shi*t!
If you want me to put up a picture of the Shim so you know what I'm getting at I'll dig one out for you as I still have four brand new shims in a packet I've not used yet!
I'm dumb founded you managed to seperate the spray nozzle from the injector body?
Can you show me a picture of the injector without the nozzle just for my collection of weird shi*t people get themselves into