Alexis Powell
Natural history enthusiast

Yes, that's a good solution if you like the 200PL (=3157N) plates. I have the 323 base on a window mount, and a 700RC2 head on a spare tripod, so I still use them sometimes but generally don't like them as much as others because the rubber base allows some wiggle, the bolt always seems to loosen over time, and they aren't non-rotating unless modified or used with something that accepts a video pin.
With the respect to the sloppy fit you found between the Gitzo GS5370C plate and Manfrotto 500 head, I have to admit that I've never used that combination (not a big fan of that plate for the same reasons as the 200PL), but I suppose I can imagine the problem. One of the flaws of the Manfrotto 500 head is that for all of its beefy robustness, it doesn't connect with the plate very solidly (or at least to the standards usually found with Arca-Swiss type clamps and plates) because the contact area along the clamping side is very small. The plate isn't gripped along its length on both sides, but rather is forced against one side of the slot by the small clamping area. The even heavier 502AH head (which I have) may be a tiny bit better in this respect, but if so, not by much. Unfortunately, Manfrotto makes the problem even worse by making the slot for the sliding plate quite a bit wider than the plate, so a short plate is prone to rotate a bit unless clamped very hard. The Sirui clamp (e.g. on the VH-10X, which I use most) is much better, both in clamp contact area and precision of fit, but it is still not as good as what most Arca-type clamps achieve.
--AP
With the respect to the sloppy fit you found between the Gitzo GS5370C plate and Manfrotto 500 head, I have to admit that I've never used that combination (not a big fan of that plate for the same reasons as the 200PL), but I suppose I can imagine the problem. One of the flaws of the Manfrotto 500 head is that for all of its beefy robustness, it doesn't connect with the plate very solidly (or at least to the standards usually found with Arca-Swiss type clamps and plates) because the contact area along the clamping side is very small. The plate isn't gripped along its length on both sides, but rather is forced against one side of the slot by the small clamping area. The even heavier 502AH head (which I have) may be a tiny bit better in this respect, but if so, not by much. Unfortunately, Manfrotto makes the problem even worse by making the slot for the sliding plate quite a bit wider than the plate, so a short plate is prone to rotate a bit unless clamped very hard. The Sirui clamp (e.g. on the VH-10X, which I use most) is much better, both in clamp contact area and precision of fit, but it is still not as good as what most Arca-type clamps achieve.
--AP