Doc I have to disagree with you.
There is a conversion kit that mounts a rack and pinion to the straight axle, and includes a solution that couples it to the steering column. Its been written up in at least one magazine, and I've seen it available for sale on the vendor's web site.
I'm not saying I think its a good idea, but the kit is out there and it appears to work.
I also don't see how you could mount a rack on the frame and not have a wicked bump steer problem, unless you mount the rack unconventionally, and use a drag link arrangement to only connect to one side, and then using a tie-rod as is common on these old trucks.
Otherwise what happens when you kit a bump with one front wheel and not the other? The two sides will be affected differently, and will turn different angles.
