The problem is that blank_str = str_create("#length"); creates a string of only 7 characters long. #, l, e, n, g, t, h = 7. The "#num" syntax only works with string initialization, and only with literals.

You could try:
var length = str_len(my_str);
length = 175 - length;
int i = 0;
for(i = 0; i < length; i++)
{str_cat(my_str, " ");}