Loading Balancing ADSL/3G on a Vigor 2850vn

Hi,
I have a Draytek Vigor 2850Vn and have ADSL on WAN1 and 3G on WAN2 two.

What i would like is the following:

Everything (All port, ips, protocols) runes through WAN1 (ADSL)
as well as HTTP port 80 running through WAN3 as well.

So, id like to load balanced normal web traffice through both (so if the ADSL line is being highly utilized then the 3G will give it more bandwidth)

But, if the ADSL drops out, all the non-TCP port 80 traffice will fail but normal web browsing will continue.

This is because, 3G is expensive, and I only want to use it as a fail back for browsing when the ADSL is down or over utilitied.

Is this possible? I tried some of the load balancing setting in the vigor, but when i pulled the lead on the ADSL, my torrent stared using the 3G.

Thanks for any help
Guy