Much has already been written about the benefits of replacing rear lever arm shocks/dampers with tubular shock absorbers so I will not repeat it here other than to say it is absolutely correct. Out of all the modifications I have made to my Morgans the biggest single improvement to handling has come from this modification.
There are many kits available to allow you to make this modification, including a factory kit. However if you are handy with a welder, it is relatively straightforward to fabricate and install your own conversion. Here is the conversion I designed for my own 75 Plus 8 and recently installed in another Plus 8.