Keep the Stock Drums? or add disk with headaches?

If drum brakes are set up and adjusted properly they will work fine. The biggest gains by going to disc brakes are 1) less fade with heat and 2) less brake pull due to mis adjustement.
On my 55 Special, I used the 45 fin drums off a 1964 Electra. I also used the roller bearings and made a spacer for the inner bearing race.
These brakes work very well and because of the finned aluminum, they dissapate heat well and don't suffer from fade much at all.

My wifes 54 Chevy does have fornt discs though. This is due to one drum being oversized and poor braking due to that. The cost of new drums and a dual master put me in the range of a new disc kit from ECI. Plus it is a car that she drives and the three kids ride in regularly. Although I think drums do a good job, the reassurance of discs with the family on board is well worth the money.

However, even if the guy(s) at Scarebird have not put this kit on this vehicle, they should know what master cyl will work with that disc conversion kit. Making the brackets for a kit is only one of the key pieces in making a car safer. Infact, if the disc conversion is not designed properly, it can be less safe than the factory drum setup.

If you have the brackets, calipers, rotors etc... for the kit, I would spend the time/effort to install them. Find out what the calipers fit, then get the matching m/c for them (the bore diameter is what you really need to make sure is right), then make sure the rear wheel cyl bore is the same as what the calipers/master fit. Finally, make sure to use residule valves for the lines.
I think the rear wheel cyls take about 2psi and the front calipers about 5psi-but I can't remember the exact pressures. The m/c I use from ECI has an internal residule valve for the fronts, and only needs an external for the rear.

Unfortunately ECI does not offer a kit for mid 50s Buicks at this time.

 

Use 2lb for MC's that are mounted under the floor, this prevents siphoning, and 10lb for drum brakes, this keeps the cylinder pistons out against the springs. Never use an MC with a residual valve in the disc brake end, this will cause dragging of the pads. Generally speaking manual brake MC's should never be more than 1", power brakes can go bigger. BUT, remember, pedal ratio plays a huge role in what you can effectively use. If you have a poor pedal ratio manual MC's may not be allow you to generate adequate pressure with the bore size dictated by your caliper and wheel cylinder bore sizes. This is why I dont think you are being fair to yourself to just throw it out here on the HAMB and expect a nice canned answer. Research.