Everything I have always heard is that the money is in the head work when it comes to unlocking HP in L Series engines. There are 300 hp race engines still on 2.8 liters.
No problem, not quite the size you were looking for but sure they will fit correctly. The 50mm tall ones should fit under the tall type k&n filters also.
You can still get them new from OER. http://lupinusflower.ocnk.net/product/22. They are about 30 dollars each + shipment. You want the Dia.44(45)mm-50mm. The other option you have is the techno toy ones. https://technotoytuning.com/nissan/240z/velocity-stacks-carbs 270 for the set.
I purchased mine directly from Yahoo japan through a proxy. I used jauce.com but there are other ones with lower fees. I paid about 100 dollars per wheel for shipment to the U.S. It took about a week for the wheels to get the proxy warehouse and about 4 days for them to go from Japan to my front door. After the fees and exchange rate I got a set of 16x9s for a little over 1300 dollars. The 16s and 14s are cheaper on yahoo auctions. They go for about 45000-55000 yen per pair of rims used. You can get new ones for 65000-70000 yen per pair. The dollar is strong right now also which helps.