U-joint on steering shaft: float or bolt down?

Not sure I agree with you on that. First, the steering shaft doesn't move up and down along its axis unless you crash and collapse it, the steering shaft is solidly fixed in place by virtue of the two snap rings and the upper shaft bearing. In the event of a crash and collapse, the column crumples and the shift tube (on column shift) and steering shaft collapse inside themselves telescopically; and they don't move away from the steering box, rather toward it. BTW, if your plastic sheer pins in your column are gone, you should not be driving the car. Mine is a '67 and they are in perfect condition.

Second, I don't believe the steering box/k-frame flexes sufficiently to pull the splines out of the U-joint. There is about 1 inch of the shaft's splines inside the U-joint. Is there really more than 1 inch of flex/movement along that axis? At most it might be measured in some fraction of an inch. Oh, I know, I'll clean out my old pot coupler and check the wear pattern of the shoes, that'll tell me how much play there was in my system at least. I welded and gusseted my K frame, use a Borgeson box and the Firm Feel sector support, so I would suspect there's less play in my system than factory setup.