gti-r mainfold will bolt onto the head no problem, it spaces the turbo further away from the block and a little to the
left iirc. So there will need to be some plumbing modification (dump, intake pipes)
Injectors, any of the sr silvia ones will fit, infact any sr20det ones will fit bar the gti-r ones (as i'm sure you know new injectors = tune/managment upgrade)
hmmmm intercooler wise, i'm not sure about the gti-r kits fitting. I know that they actually go under the support beam on the gtir's as they don't have holes from the factory. Unless you're planning on running a monster turbo 3" intercooling piping is way to big imo. I would go no bigger then 2.5inch unless chasing big numbers. Infact if it's only a mild turbo upgrade the stock piping should be fine. most t28 frame turbos have around 2inch outlets.
Bring on the questions if you have any more!