Actually, the hidden hinges on my '28 are totally scratchbuilt. I bought 4 coupling nuts, 1/2" thread, and drilled them out with a 1/2" drill to allow a bolt to pass through. The hinge box is a section of box tubing that fits the coupling nut length on the inside, and the hinge arm is nothing but 1/4" x 2" strap
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The most important thing when doing hidden hinges is to mount them properly so the door does not bind. For this you need to determine how far apart they will be, and purchase a steel rod of the hinge pin diameter so you can put your hinge parts on it and fab up the hinge mount from plate or box tubing. This way you know the pins are inline as they should be. Mount the plate or box tubing inside the jamb to locate both hinges at once, put them in the closed position, and measure from the outer skin to the center of the hinges, top and bottom. Duplicate these 2 points on the doorjamb, mark a straight line between them, and you know how deep the door pockets have to be.
Over-engineer the mounting parts so you end up with a strong door. You don't want floppy doors coz ya went with thinwall plate or tubing, so spring for the 1/4 wall stuff. A side benefit is you can drill and tap your mount holes in the pillar mount as I did. You don't need in and out adjustment of the hinges at the pillar because you sink the boxes in the doors so the skins come out flush. Use some 1/4 plate and nake up a nutplate for the door pieces, drilling and tapping the nutplate AND the mount welded into the door at the same time. Once you trial-fit the door, and see how much to correct the door sag, just drill the hinge mount holes in the door pockets oversize for the adjustment and bolt the door on and adjust it like stock...
Doc