Well if you really want to add spaces to make up the length then you could do this ...
per = per.PadRight(16);
len1 = per.Substring(0, 16).TrimEnd();
len2 = per.Substring(16).TrimEnd();
Or if you are just concerned about that error then I think this is better
string len1 = per.Substring(0, Math.Min(16, per.Length));
string len2 = per.Substring(Math.Min(16, per.Length));