Eric Taylor down there in sunny North Carolina has good reason for erring on the side of caution. He was one of the 'pioneers' who sacrificed his Squarebird to find out what would work. We owe a lot to these folks for laying the foundation for our more modern methods.
Eric says to read the 'Disk Brake Conversion' thread. I agree. Since that thread, Squarebirds.org has opened up to include the original Thunderbirds (or ClassicBirds), BulletBirds, etc.
There is no magic pill but some setups work nicely. BTW, when I hear that someone cannot find S-10 parts, I am floored. GM produced so many millions
of S-10 and S-15, Cadillac, Pontiac, Olds, and Buick vehicles that all used the same brakes; there isn't a more common brake in existence. I don't know how many suppliers made these brakes, but it must be dozens, to keep up with 'aftermarket demand'. As I stated, my local parts store has FIVE different pads, from cheap to ceramic. Rockauto.com has seven pads; from 1977 Buick Electra to 1992 Pontiac Firebird and all the cars in between.
Ok, let's examine Ford spindles from the 'horse's mouth' Ford Cat:
Notice, from '54 thru '56, Ford shared the exact same spindles with Thunderbird. Why not a RH + LH? Because the same spindle was used for both but the SPINDLE ARM came in RH & LH, and the Arm bolted onto either spindles. Thunderbird carried this over through '57 (consistant with ClassicBirds).
In '57 - '59 Ford Cars, there is a change that Thunderbird follows in '58 - '60 (consistant with Squarebirds). What is this change? Spindle Arms are now integrated into the spindle forging and we have a LH & RH spindles. Both, Ford Cars and Thunderbirds, share the exact same part numbers.
Because Ford Cars and Thunderbird used the exact same spindles, the Scarebird bracket fits both. Scarebird has one for the ClassicBird and another for the Squarebird. The newer spindles have a new hole in the upper spindle arm.
What is the difference between Granada spindles and Squarebird spindles? Mainly the lower ball joint hole. It is smaller than the Thunderbird's, but may be reamed to spec. There may also be a slight difference in the spindle arm length or angle, but not enough to notice. I am using both setups with great success, and no issues with wheel alignment.
The real problem is finding Squarebird firewall brackets for the booster/master combination. Remember, original boosters were made for shoe systems. If used on disk brakes, they will feel like manual brakes (at best). If you are doing proper Power Disk Brakes, use a booster capable of producing 1,000-psi like a two-stage 8" vacuum booster. Then, you will have power brakes. - Dave