I am just a hobbyist, and studied brakes out of necessity for my B body. Stuck with it by studying in more detail the internals of MC. It surprises me that some of the current business that do this don't source there own parts considering how other hobbyist go an get parts made. Especially since in many cases they should have good technical documents from the period. I guess the focus on the modern work, overrides some of this. But it still amazes me.

All the variations of the B body 1 1/8" Bendix from the 67-70 and they only used 2 types of seals. 3 different primary pistons, 4 secondary, but the seals and configuration are the same. The generic market turned this into just two repair kits, and the actual MC are really only one. Original Bendix seals have part numbers on them.

Having said that, I spent a day on the internet and believe the MK 494 kit for Fords may have what you need. Why a brake repair facility could not figure this out? Frankly, pistons may even be closer then the 551.

If I had the actual original pistons like Brad does I could do the comparison. Plus document in detail the pistons. Similar to what I have done in the B body types.

Rockauto has 494 kits, maybe worth the cost to get one and see if that meets your needs. I would be shocked if Bendix made a new seal type/dimension for every 1" bore MC they made in the late 60s to early 70s. Especially from a Disc to Disc perspective.

I hope this helps the E body guys. Maybe the A body guys too that need the 551 kit.