Not sure to understand your real use case but optint was made to use 63 bits integer (unboxed) or 32 bits integer depending on the platform with the same Int32 interface (plus some functions).
The purpose of it is to allow to use unboxed integer when it’s possible for adler32 or crc32 checkseum (see checkseum).
Then, the Int63 provided by @CraigFe is young but currently used by Irmin to, again, take the opportunity of the unboxed integer in 64-its platforms.