My homebuilt steering box has a .210" T-bar in it.
Not many steering shops are set up for torsion bar replacement in the input shaft / valve. Reason why is that it takes quite a bit of tooling and fixtures to do so. The torsion bar has to be clocked precisely in the input shaft of the box, checked for function, cross drilled and pinned in place. If it isn't just right your left and right bias will be off, as in you will get assist really easily turning one way - and not so much the other, or even worse you box could "ghost steer" where with no input at all it could be trying to steer one way until it hits the stop. Typically the shops leave the torsion bar / input shaft assembly alone. That being said I know there are shops out there which are capable, but I don't know who. Check with Redhead steering.
One way to go about getting a .210" bar is to swap the entire input assembly. This requires no fuss - no muss, but you have to be willing to rock the input shaft out of the donor box.
As a general rule earlier Saginaw steering boxes came with a 13/16" input shaft, 1980's and later seem to be 3/4". That means that the late 80's early 90's wrecking yard Z28 with a .210" T bar is going to have a 3/4" shaft, not a 13/16" like your car came with. If you go this route you have to run a GM rag joint, or make one. Swapping the input assembly isn't that hard. You do need to make a spanner socket so you can accurately torque the adjuster plug, then back it off to spec. There are some Torrington bearings in there that will appreciate correct preload. Links to all the Saginaw documents are in the link below. Other option would be to have a steering shop do the swap for you.
If you want to see what I did look here.
