Well, dont give up, if you really wanna do it, try.
BTW, you're in luck, i have a K&N Master catalog (perks of working at a parts store)
Part number for 240Z 70-72: 56-1410 (these are sold individually. Only One carb) They typically run $50-65 each. So total is around $100-130
Part number for 240/260Z(flattop SU) 73-74: 56-1430 (Also sold individually, you will need two)
I do not know the price on these, but I would Imagine it is very similar.
I have a 72, bought both 56-1410's i really like them, chrome plates, decent sized cylinder shaped filter. They look really good. The only problem I had was, they forgot the studs you need to hold them together. When I called to get the studs, they said it would take one week in the mail. It took three weeks, and they sent me the wrong studs.
I forgave K&N though, I really like their filters and have K&N on every car I've owned.
There are a number of other options on good filters, search around on some Z parts sites, you'll find a few more brands.