Couldnt speak for the 4AG or 18RG, but for the 2TG I am going to use GTR fuel injectors.
Both the original and GTR injectors are low impedance. There is a difference of 1mm in the overall length (GTR is longer). This can be compensated for with some shim washers under the fuel rail mounts. The pintle end is the same diameter, but the feed in end is smaller on the GTR by just under 1mm. Might be able to get away with using a 'fatter' o-ring to seal it. If that doesnt work then will have to machine up collars and press them into fuel rail.
BTW the original injectors are 125cc/min and the GTR ones are 440cc/min.