Mmmph. Even if we can process such an order... I fully expect the line would probably be very intermittent. The lowest provisioning I'm aware of is 384Kbps - which your ~300 doesn't even reach. So it wouldn't really work. If we were speaking with a hypothetical, untested line, I might have been hopeful for a minor miracle (maybe above-average gauge cable, for example) - but that ~300Kbps is an estimate provided to you by an install tech? That's based on actual readings, not projected hypotheticals, so I'd trust them when they say that.
So... no. I don't think the line would work. If you want to try calling in to our sales department and see if they're able to place the order, I can't stop you, but I really don't think it'd work, unfortunately.