Yep, both server and client IDs are being changed into free slots,
I just changed the IDs to 650 and 651 (651 being MFFS) and ingame their was only one card, 907, which is the MFFS card, this card did not work in the ARS core, and when I tried to use it on a MFFS capacitor the game crashed.
Here is the main part of the error report if it would help:
java.lang.ClassCastException: immibis.ars.ItemFrequenzCard cannot be cast to chb.mods.mffs.common.item.ItemCardPowerLink
at chb.mods.mffs.common.item.ItemCardEmpty.a(ItemCardEmpty.java:99)
at ur.a(SourceFile:88)
at ir.a(ItemInWorldManager.java:424)
at iv.a(NetServerHandler.java:578)
at fk.a(SourceFile:58)
at cf.b(MemoryConnection.java:80)
at iv.d(NetServerHandler.java:136)
at iw.b(NetworkListenThread.java:57)
at bec.b(IntegratedServerListenThread.java:108)
at net.minecraft.server.MinecraftServer.r(MinecraftServer.java:703)
at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:599)
at bdz.q(IntegratedServer.java:159)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:497)
at fy.run(SourceFile:849)
From I can tell from that error report, even though the IDs of the cards are different, MFFS is seeing the ARS card instead of its own.