Love my frankenpens-fitted jinhao x450 with zebra G and jinhao 750 with Nikki G. Nikki G doesn't flex as beautifully as zebra G. X750 with Nikki was an extremely tough fit. Took me almost an hour with sore and red fingers but I was so happy when I finally got it fitted nicely after reading online that this is an almost impossible feat. But then I subsequently broke the nib by jamming it too hard into the cap. GAH!
The x450 although wonderful to write with at first begins to give in to railroading, or leaking, or completely refusing to ink up. Sigh... but still, when it works it's a joy to write with.
Noodler's featherproof ink seems to jam up the pens, pelikan ink is completely shitty (so diluted!). So far, diamine ink seems to work best.
Had to re-fit both pens. Thank God I bought 10 nibs this time round. Strangely fitting both were a breeze, unlike the first time with the x750. Also managed to stuff the nibs and feeds deeper into the pens, thereby minimizing bruising by the cap. 