Wanting to change over to the rack on my 64 and 73. Is there factory units (any brand) that will work or just get one of the aftermarket ones? Pros and cons of each.

Just for clarity, are you working on the rear differential (ring and pinion gears) or steering gear?

Ring and pinions available now are all or almost all aftermarket. Dr Diff has a good selection.

Firm Feel is probably best bet for a high end reman steering gear. The parts store offering is often sloppy.

The only kit I am aware of that will put rack and pinion steering in a stock K member is Unisteer, crap.

Otherwise, you are looking at replacing the entire suspension to do that.

I would go with a FirmFeel recirculating ball setup first.

You're going to want an aftermarket K-member with the appropriate hardware for this conversion.

As mention, FirmFeel upgrades/rebuilds offer a nice drop-in improvement.

We went with a magnum force transformer on our E-body.
It based around the stock style spindle , which allows you to run stock brakes 11.75 etc. and if you have upper tubular control arms they can be used.
It was one of the cheaper ones we priced , $3500 complete with power rack and pinion and double adjustable coil overs.
Motor mounts and sway bar are options.