Custom header order

I have fumbled around and managed to get a header that I wanted
inserted. I was hoping to insert a header which could save time in
tracking down spammers if one used auth to send mail through us.

Here is my mc line:
LOCAL_CONFIG
dnl Add an X header
H?F?X-Authenticated-Sender: $.$?{auth_type}<$?{auth_ssf}${auth_authen}
$.>

Which produces this header if the user authenticated and does not
insert the header if they don't authenticate:

Received: from mx0.ispdone.com (mx0.ispdone.com [65.90.177.14])
by mailhub0.ispdone.com (8.13.1/8.13.1) with ESMTP id lBQ2gP2c015871
for ; Tue, 25 Dec 2007 20:42:25 -0600
Received: from mx0.ispdone.com (localhost.localdomain [127.0.0.1])
by mx0.ispdone.com (8.13.1/8.13.1) with SMTP id lBQ2gJjF028245
for ; Tue, 25 Dec 2007 20:42:19 -0600
Received: from smtp-auth0.ispdone.com (smtp.ispdone.com [69.39.47.46])
by mx0.ispdone.com (mx0.ispdone.com [65.90.177.14])
id jBOKgI0361243804ev ret-id none; Tue, 25 Dec 2007 20:42:19 -0600
X-Authenticated-Sender:
<<<<<<<<<<<<<<<<<<<< Received: from [192.168.12.102] (pool-69-39-37-45.texascom.net
[69.39.37.45] (may be forged))
(authenticated bits=0)
by smtp-auth0.ispdone.com (8.13.1/8.13.1) with ESMTP id
lBQ2gEE6031230
(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO)
for ; Tue, 25 Dec 2007 20:42:15 -0600
Message-ID: 7010106@ispdone.com>

I have two questions.
1. Is this a bad idea to begin with? Is there something about adding
this header which may somehow interfere with delivery or have some
other bad affect?

2. I don't like where sendmail is placing the header. I would like it
to go below all the "Received:" headers. Maybe above the "Date:"
header. I can't seem to affect the order. Is there any way to relocate
the header?