I owned a fully restored 1966 Daimler V8 mark 2 , several years ago.
I did most of the restoration myself over 2 years.
I remember that to get the column gear change in exactly the right spot to correspond with the column change marked positions, required a lot of under the dash adjustments.
There are several adjustment rods , with a cup and ball on each end , for infinite adjustment. It took a long time to get it just right.
I assume that your jag has exactly the same setup hidden under the dashboard?
To alter this to respond to a floor mounted gear-change would be the most difficult job ever, (in my opinion).
Perhaps if you joined a jag forum and asked , you may find a person that had even done the job before?
That may be your only hope!
May the force be with you...
Peter