What size injectors???

for 325 rwhp, 19 lb injectors will be fine. If you start trying to build up over that your gonna need bigger ones. I would at least go with 24 or 36 lb injectors. If you think a S/C is in your future, then go with 42 lb injectors.
How are you planning to run the N2O?

24's(if u r close to done) or 30's(if u plan on going with more power)

Just my opinion, but I don't know if I'd want to put down 325 rwhp on 19# injectors. The nitrous will probably feed off the rail (I'm assuming) and might have a small impact on fuel pressure as well (not sure about that).

I just installed 30# injectors based on my plans and spoke with Tim @ T&J. He said they put down 415 rwhp with 30# injectors in one case. In his words, it was taxing the injectors, but still safe.

If you are planning to do a substantial buildup, I'd say get the MAF, tune & injectors for whatever you're looking at ultimately putting down. You can always continue to tune for the power level, but it could get expensive recalibrating the MAF & replacing the injectors every step of the way.