Tube shocks are not an upgrade, they are a change. The design of the MGB does not allow an easy change to tube shocks without potentially dramatic side effects. Most (not all) people who convert to tube shocks with the usual kits end up changing back due to the hard ride they encounter. The reason for this is that there is little room for mounting tube shocks at the angles they were designed for. With a lot of experimentation and modification, tube shocks can work just as well if not better than levers but the potential gains are not worth the time, money and effort in my opinion. If you are the type who likes a challenge and enjoys fiddling with your car, then perhaps tube shocks are for you. If you simply want to install a set of shocks and have everything work properly out of the box then use the factory design.
Fronts are another matter altogether. Since the front lever shocks on most M.G.s also incorporate the upper suspension arms, there is no "good" way to add a tube shock without replacing parts. The usual tube front conversion is to a coil-over setup. Moss is selling two shock kits (shock conversion only, no suspension parts) for $510 and $600. With suspension parts their price is $2000. At this point you're spending a lot of money and changing the character of the car, so it's up to each individual whether or not this is a good path to take.
In good order, lever shocks work perfectly well for 98% of MGB owners. Problem is, after 40 years most tube shocks are not working properly. It amazes me that so many berate their ill-functioning lever shocks which have been working hard for many decades, yet don't blink an eye at replacing five to ten year old tube shocks on their modern car.
The best thing you can do, in my opinion, is install a full new set of remanufactured lever shocks from Worldwide Imports. They work as well if not better than new originals, and come with a lifetime warranty against leaks or defects.