Create blockpage packet with all current headers, add unicode string of html, change IP length, change seq and ack, recalculate checksums and send packet to adapter.
Instead I would do the following:
1) Wait for a response packet from WWW-server
2) If HTTP response packet contains a censored word then modify HTTP packet to contain a redirect packet like:
<html>
<head>
<META HTTP-EQUIV="Refresh" CONTENT="0;URL=http://www.censored.com/">
</head>
<body>
</body>
</html>
This simple redirect packet in most cases will be smaller in size than HTTP server response and you can add padding string between to exactly match the size of the original packet. This way you won’t need to affect SEQ/ACK fields, just recalculate TCP checksum.