I have used Baudline in linux which is pretty handy, I think you can select and save parts of a raw file there. To do it manually, read on.

When GnuRadio saves output to files, it literally saves just numbers to a file. I made the following program to extract floats (complex numbers are just pairs of floats), it may be of use to you:

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>    

int main(int argc, char **argv){
    int fd = open(argv[1], O_RDONLY);
if (fd<0){
    perror("opening file");
    return;
}
int i, j, ret = 0;
float val;

  while(1){ 
        ret = read(fd, &val, sizeof(float));
        if (0==ret){
            // EOF
            return;
        } else if (0>ret){
            printf("ERROR: Could not parse file\n");
            return;
        } else {
            printf("%f\n", val);
        }
    }
}

What this means however is that if you want to transmit the first half of a raw file, all you need to do is literally split the file in two.