the best for you money would be to get a complete L28, a good head such as a N42 or P90, get your carbs re-calibrated so that they can flow enough for the bigger motor, and a full exhaust. This should give you a big upgrade in power and torque.
I have a 280ZX which i have just swapped an RB into and also a 260Z with a L28 in it.
BTW - where are you located?