So far, got:

Open "filename" for binary access write as #1
Put #1, 103, "345"
Put #1, 132, "some text"
Goal is delete rest of line after the "some text". Obviously I can compute the position and the line is terminated, but didn't see relating info in the help system.

Paramount is speed, don't particularly want to read, edit and write a file as seconds count because they are part of a repetitive task in the 1000's.

Barring native solution, is there a 3rd party toolkit?

TIA
Grug