I would have thought you could hold the block in the 4-jaw and turn a spigot on the end. At the same time drill the end for a suitable stud to take the toolposts nut.

Alternative would be to put a bigger hole into the top of the block and incorporate the spigot as a larger diameter section of the stud

A picture would help.

Prior to this change might I suggest a little experiment. Attach a dial gauge indicator to the bed with the stylus vertical on the corner of the compound slide where the tool is usually positioned. Apply a downward load to the corner and note the deflection. Repeat with the stylus on top of the chuck again applying a downward load to the chuck. The first value should be very small, I get less than 2 microns. For the second I get 15 microns for an estimated 5-10 kg load. If your first value is not very small try with tighter gib settings. If things are in good order the weak point is the spindle stiffness rather than the slides.

Also worth applying an upward load to the chuck. Deflection should be similar to the downward, if more, the headstock bearings may need tightening. I found significant slack had developed with a few years use from new.