You need to store the birthday (50 bits, rounded up to 64 bits) and the nonce, 26 bits rounded up to 32 which is 12 bytes.

If you got fancy you could use 76 bits or 10 bytes rather than 12 and save 16% at the expense of some computation.

The current algorithm uses

64bits + 32bits -> 96 bits

Only 50 bits of the 64 bits are relevant, and only 26 bits of the 32 are relevant, so that's a total of 20 you can shave right away.

However, with current implementation 22 bits of the 50 can be immediately inferred from the location in the hash map, so there's another 22bits saving there too . . so a grand total of 28+26 is required - 54bits . . there'd be a little more calculation for all the bit-shifting, but not much. The algorithm could use as little as 432MB without much additional calculation and without loss of collisions.