You can leave your existing MC in place and then install an inline adjustable proportioning valve somewhere between the MC and the rear brakes (which you will eventually have to do anyway) like this one:
http://www.hydratechbraking.com/mast...7DE11142513E51
This way you can adjust what the rear brakes will be doing by comparison to the front drum brakes. It will be drivable, but not optimal, suggesting you get the front brakes updated ASAP of course...