Have you improved your front brakes yet? Improving your front brakes will go a long way to improving braking, prior to doing the rear swap.
First, drum brakes get an advantage over discs since the friction material is Wedged into the drum. This means for the same force you get additional braking power with drums (to a point).
A couple of points on this. Rear brakes only provide abou t20% of the braking force, and front brakes about 80%, so I think it's not an issue in that sense.
They don't need much force unless you are trying to lock up your rear tires. Nevertheless, you could tune their effectiveness with an adjustable brake bias valve, or by using different brake pads. The rear disks may not give you any improvement in braking distance, but will give more control or feel.
One thing that makes drum brakes bad is that they tend to stick, or not release braking force the instant you let off the brake. As you say they are wedged into the drum. Sometimes, I believe this is called " they are self energized".
As for the " rear brakes are applied first " problem. You can probably gut or remove that valve from the system. If your rear drums are properly adjusted, they should be dragging slightly on the shoes.
So you might be better off just upgrading your front calipers and rotors to a newer version, if you're primarily concerned with stopping distances rather than feel, control, repeatability.