The RFC you quoted actually refers to line lengths within a MIME message as a whole (headers and body), and only after Content-Transfer-Encoding has been applie

The RFC you quoted actually refers to line lengths within a MIME message as a whole (headers and body), and only after Content-Transfer-Encoding has been applied, not before. In other words, your text parts can have lines of arbitrary line length, provided you encode them as QuotedPrintable or Base64 and set the Content-Transfer-Encoding header accordingly, which most email clients do.