They fit but they may not be long enough. I converted a p360 to ball top using a Wico handle. The handle was about 1/16" to short to get the C-clip on. I had to sand down the actuator to make it work. However, it works well. You could probably also shorten the metal spacer.
That's curious. I've only converted (4) Happ P360's with Wico handles so far, but all of them fit perfect without doing anything. The spring couldn't compress enough to put the retaining ring on yours? IIRC, on mine the spring compresses ever so slightly more than with the original Happ handles, but not enough to notice.
Are those repro handles, or original Wico's? If they are repro's, perhaps the groove isn't in exactly the same place as the originals. Just a thought???