Here's what I would do:

• Open up the text file and the HTML file for reading.
• Open up a temporary file for writing.
• Read from the HTML file (in reasonable sized blocks), up to the point you want to insert, and write each read block to the temporary file.
• Read from the text file (in reasonable sized blocks), and write each read block to the temporary file.
• Read the rest of the HTML file (in reasonable sized blocks), and write each read block to the temporary file.
• Close all files.
• Move the temporary file to the HTML file.
• Eat a slice of chocolate cake for a job well done.

Abigail