Will,

My unprofessional take on this! If the piston is the proper one for the master cylinder, and your Ford piece has the same physical dimensions as the original Chevrolet rod, it should work perfectly.

I think for the master cylinder to function properly, the piston need to retract fully. If your makeshift part allows this to happen, your bleeding problems are elsewhere.

Mixing and matching brake parts would be the ultimate NO-NO, but it sounds like your Ford items are only linkage related.

Bleeding can be tough sometimes, so it is probably a procedural problem as opposed to a mechanical problem...we hope.

I'll bet someone will pop in soon with more specific info and maybe even a lead on a replacement Chevrolet part.

Stuart