I've always heard it's not wise to have disc in the rear without first or also having front disc.
Yep, Front brakes do most of the work, so if you have discs on the rear, they're likely to lock up even under moderate braking. You don't want to be swapping ends when some yahoo pulls out in front of you. Like Jerry said, it's probably going to be tough to get setup correctly.
I have a friend with a '63 Corvette and he's replacing the rear trailing arms and installing rear discs but can't afford to upgrade the front drums yet. He talked with Willwood and they said they see that all the time for the same reason as my buddy. They say to put a proportioning valve on the rears and spend about a day setting it up so the braking force is equalized.
