first of all, you have a V6 3.0L engine. it makes roughly 160 HP and about the same for torque. I don't know if you can fit a small block in that engine bay, but you could do some mods to your six and you will have a good amount of torque.
1. Get a K&N cold air filter kit
2. Get a good set of headers, cats, and muffler
3. Shave the head 1mm to increase compression
4. If you are going to shave the head, see about getting some oversized pistons and bore it out.
These things would give you more torgue and punch off the line.
That would be about the same amount of money or cheaper and still have your original reliability and equipment. I have only seen V-8 kits for the 70-83 Zs.
I have a 84 300ZX with no motor, & on hand Ford V8s & SBCs with AT or
sticks, which ever is best/easer.
The V8s are carb moters, which will sempifly wirying.
The 84 ZX is a 5spd so I can go with AT or stick.
I have seen it done and the engine fits very well !!! I don't know if the stick will line up right so I recomend auto... acutally the 300zx I saw had a 350 in it and then he upgraded toa big block and still retained the stock body.. no holes in hood(as far as aI know.. haven't seen it in a while) He fab'd everything thing up byhimself... last I heard he was runnin 14's with traction problems and a bad tranny(would shift below 4000Rpm) he used the stock rearend and was useing drag radials I believe(it's been a while!).. go for it.. it's a very uncommon and eye opening swap.. it will definitly be a sleeper...