i have a 260z and i was wandring if i should do a nissan v6 350z engine or a chevy v8 350 small bloc engine, and how much a v8 will affect handling just need some advice

It's all been done. You will find more info on that sort of stuff over at www.hybridz.org. Most of us here are into keeping things a bit more on the "classic" side.

I see, sorry

V6 Nissan from a 300/350 = the sucks to install

V8 from Chevy, = slides in fairly decent with just a few modifications

LT, LS, Anything else V8 from Chevy really.

I think that Nissans I6 (RB) and I4 (SR) are fairly easy to put in (though a little more work then the V8's), and can take a lot of power stock. Though I hear that the SR motors have poor handling because the motor stops right in front of the wheel wells, so there is no weight on the front wheels to help them grip in cornering.