You need to set the Pop/Opening Pressure on the Injectors because none of the Springs will have the same tension on them and the dimensions of several of the parts inside of the Injector that compress the Spring are not all the same size.
I also do not think a Monark Nozzle verses a Bosch Nozzle will have exactly the same dimensions either as far as compressing the Spring goes.
when you get your new Nozzles remember that the 2 parts of each Nozzle are mated together. Keep all of the parts of one Nozzle together.
Also, keeping all of the Parts that come out of one Injector together makes it easier to set the opening pressure because it will be closer than if you mixed the parts.
There is also 2 different types of Injector Bodies. While the 2 different types of assembled Injector Bodies can be mixed on the Engine the parts of the 2 different types are not interchangeable and cannot be mixed together.
How to Rebuild Injectors
http://vincewaldon.com/index.php?option=com_content&task=view&id=20&Itemid=28