The aftermarket brackets are not made of the same kind of steel as the originals and are therefore difficult to make usable. Another issue is that it is difficult to attach the brackets to the channels. They have two prongs on them which are installed like a staple.
It is also difficult to reuse the original brackets because the metal is stiff and brittle, making it hard to straighten the prongs out.
It is much easier to hold the channels in place with tiny self-tapping screws, and you only need a few of them to keep the channels in place.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission