If you can find a head that is functionally sound, then just bolt it to your block and fire it up. See what happens. I can't be specific as to what might happen, obviously the hope is that you either burn less or no oil at all. Whatever does end up happening to the engine will give you a better idea as to where that gremlins hiding. The caveat to this is that the head needs to be perfect for a real diagnosis. You can buy a rebuilt head from Datsun Parts llc. for... Ehem...$450 but at least you'll know it's not the head
If it is the rings (or the lower end at all), then you're staring down the barrel of an engine pull and rebuild. May as well get an F54 from a 280zx if you're gonna stay with the original engine...
If you want to swap a V8, I would suggest that you be aware of how much weight is going in the car. An LS engine has an aluminum block so you save weight and add power all at once, but unfortunately that's something only available to those with a lot of cash to burn (if that's you, go for it) or some very good connections. Iron block LTs (the JTR kit) are just that... Big, heavy chunks of cast iron that love to screw up your cars handling. I've never been in one personally, but a friend of mine who owns a Z shop says that anything with an iron block doesn't belong in a Z. That includes the l28. He builds cars for road racing and handling so he might be a little biased toward lightweight hardware, but to me there's no sense in having 500hp if it's all going to go into heating up your brakes because you can't make it around a corner.