Your first post says you were thinking of dual, power, disc. Then it sounds like you purchased dual, power and are staying with stock drums. Assuming I got that right, and you bought from a well known vendor, they would be the ones who would advise on residual valves, If you told them what you are doing they should have sent the necessary parts in the kit (IF you bought a kit).The kits come with different valving depending on drum/drum, disc/drum or disc/disc.
Just remember, there are three components we are talking here. The dual MC is for safety, the power booster is for brake pedal pressure and the disc is for safety/quicker stops. Put them all together and you have a great safe combo that will stop quicker with way less foot pressure. Dual Power, drum brakes are not all that great of an improvement in actual stopping ability. I don't think you will notice any change except foot pressure. Boosters are kind of for disc brakes anyway. You will really, really like the Power front disc brakes. You will find a great improvement as Cletis and others indicate. I have done that kit on several TF trucks.
Residual, or check valves, are used to keep slight pressure on drum brakes because the return springs will pull the shoes too far away from the drum unless you keep adjusting the shoes. Also fluid can't return to the MC with a slight pressure always applied. They are used on disc but meant for drum. Different pressure values are used for front and rear.
The proportioning valve is used to apply less pressure to the rear brakes. They require less force than the front and, as said, the front do more stopping than the rear. (weight and weight shift)
The residual valve on discs is a lower pressure than for drums also, so that you don't essentially lock up the front brakes.
