I have seen some front end conversions that looked pretty good. But it takes A LOT of work, A LOT of money, and A LOT of custom metal fab to get it to all end up right.
Even the kits you can buy need a lot of work to massage them into place so they don't look like balls.

Last conversion I saw that actually looked good, had a price tag of over 10K. 10K in body work alone just to make one car look like another car. I don't know about you, but being "different" isn't worth 10K in my book. It boils down to the fact that you just spent 10K on a 240sx and still have a KA24DE that runs like shit, and makes shit for power..... and it is still a 240...
If you don't want to spend that much money, then buy one of the conversion kits and try to get it to fit the best you can, spray the whole car a matching color, (a more flat black tends to hide the crappy body panel fitment) and then be happy with what it is. But you're still looking at spending 5 grand. And you still have a crappy running KA in there. Personal advise, and this is big coming from me considering my "history" abandon your endeavors and spend your money in much wiser places.

If you want a skyline, then buy a skyline.

If you can appreciate a GTR the way it should be, then buy Blackzenki's GTR as it is one of the best examples of how a skyline should be. It is seriously worth knocking over as many liquor stores as you can find to come up with the measly 20 grand to buy it. It is seriously worth so much more than that.