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You can fit a ka24 throttle body to your 300ZX spec vg30 intake manifold. The ka24 unit is 60mm vs the stock 54mm. The specific one you need is from an 89-90 single cam ka24e 240SX with cruise control. It bolts right on. The exit is larger, so porting your intake manifold would be highly advisable.
The stock z31 upper intake manifold is horribly restrictive. You can gain a huge amount of power with a better flowing manifold. Some people have cut open, gutted, and re-welded the stock manifold. Others have fitted the much better flowing pathfinder manifold.