I think the six-cylinder won't just slot in. Probably would have to move the radiator in from of the panel.

For "a bit more power" just use an SR20DET -- you'll get more power than a V6.
Nissan VG30 V6: 160 HP
Nissan SR20DET: 200+ HP

lol at knowing about datsun motors- i always wondered what the B was for, know i know and stand corrected.

I would forget the inline six cylinder completely. While it should bolt up to the gearbox OK, it's at this point that the good news ends.
The engine is longer, two cylinders longer. It is heavier, & extra weight never really regarded as a performance improvement. The extra weight is right up front, which rebalances the weight distribution in a negative way.

The extra length necessitates additional mods up front, such as radiator repositioning & possibly even structural mods. Again, not really the best solution.

Most cost effective way to restore the car to service, .... repair your engine with a serviced head.

More performance? Soup up the L20B. They respond quite nicely to performance improvements

Even more grunt? Nissan provided a number of 4 cylinder engines such as the CA18DE[T], SR20DE[T], FJ20DE[T], & the Z series that will fit the engine bay very nicely, don't weigh too much, will keep the weight distribution within reason & can provide enough grunt to scare you sihtless.

Think thrice, act once.

Most of them won't break the bank either.

ok ill try and track down a L20B engine. out of curiosity how could i tweak the L20B to give it a bit more power?

same way you tweak any other engine. $$$ or hard work -- your choice.

Doesn't one of the Z series heads bolt straight on, and they're cross-flow so more bang for ya buck ?