That's not possible because the base 64 encoding is longer than the plaintext. There are 4 characters of text in the URL for every 3 characters of text entered.
If you're willing to jump through hoops and js-compress your content, you can deal with this issue, unless your compressor uses something like crc. That way, you can grow the plaintext faster than the b64 - though you'd still need to do clever things with the url.