70MB html document? ouch
Not sure what you can do other than editing it or writing a small program that loads it up in a stream and modifies that one line. Loading all that into memory is going to be large. I've never used HAP on that large of a document, unsure how it will handle
Another thing you can do is add some code to HAP to look for that 6" as it's parsing and ignore it/change it.
WordPad does a decent job of editing large files, it might take a few minutes to load but I've had it load multi-hundred megabyte files before.
But if that's just the head, I'd be afraid of what the rest of it looks like