I've got a 3315 and picked up a 501 sickle this summer. The 3-point connection and PTO connect just fine. However there are a couple clamps on the sickle that are intended to attach to the lower lift arms. The one on the right, the sickle side, doesn't seem to do anything and can be left loose. The one on the left ties in to a rod that causes the sickle to pivot up when the lift arm rises. Handy for getting over obstacles or tight places. The 501 seems to be designed to cut only when horizontal, when lifted the drive angle changes such that it would likely break the wooden arm. So stop the PTO before lifting.
For me, the left clamp contacts the stabilizer right where the lift arm attaches, so I can't get it to fit. But the clamp appears to be modified. So I just disconnected the clamp at the mower. It works, the downside is that the sickle doesn't fold up when I lift the three point.