Lecat. I have a friend that has a '99 4/4 Sport the same as yours. By comparison to the new models the suspension is quite "firm" and the steering is much heavier. In fact his wife now is not so keen on taking a trip in the Morgan because of the ride. Fitting the rising rate suspension will make a huge improvement as will fitting decent dampers at the rear.
To convert yours to rack and pinion steering would be quite expensive but I would suggest you change to roller bearings and remove the damper blades at the same time that you fit the rising rate springs. This will lighten the steering and the cost will be minimal if the work is done at the same time.
http://www.gomog.com/allmorgan/rollerbearing.html You would also benefit from fitting 4 leaf rear springs which are much more compliant than the 5 leaf springs that would have been fitted in 1999. You should be aware however that there have been some questions over the quality of the rear 4 leaf springs used in production in the last year or so. My car is currently fitted with 5 leaf rear springs because the standard 4 leaf fitted did not give me sufficient ground clearance. I would like to fit better quality 4 leaf springs to get a more compliant rear suspension. Fitting Morgan's 4 leaf springs would also have the effect of lowering the rear of the car which may not be such an issue with the taller tyres on the 4/4.
PS. I am not 100% certain but on re-reading the information that I have it seems that a conversion to roller bearings is part of the kit.