The rears not locking up is irrelevant. Dissimilar brake systems require a proportioning valve as disc brakes need higher pressure to function than drum brakes.

That's why I used the 'pushing rope' analogy - your trying to move something that requires 1000 lbs. of force with only 1 lb of pressure. It simply won't work.

A Proportioning valve and proper Dual Circuit master cylinder will give your car brakes.

You could always get an adjustable proportioning valve. That way, you can adjust it now for your disc/drum combo, but then later when you go four-wheel disc, have the added luxury of being able to adjust your front/rear brake bias.