Hello Arjen
Before I go into some technical details , I am going to ask the question why you want to do this? A lot of the character of the early +8 is encapsulated in the gearbox , that may be one of the reasons they are referred to as "Moss Box +8s"
As well as the character the fitment of an non-original box will destroy the value of the car , and render it illegible for any historic racing. The Moss Box can be built by Billy Bellinger into an all-syncromesh affair (at a price), and I would strongly advise trying out one of these first.
Coming from other pre-war cars you must be familiar with double declutching, and used this way the Moss Box will give you a very good (if not exactly delightful) change. True it will never be a rifle bolt action, but this is part of the skill set you need to drive this car well.
If you want to change boxes then by far the easiest route is to use the LT77 , or R380 from the later +8s. The latter will cope well with the additional torque of the 4.6 engine , the former less so. However I would strongly advise against the 4.6 route unless you want a "Drag Racer" . A fully modified 3.5 litre engine will produce 250 to 280 bhp and be nicer to drive , more balanced handling, and still eligible for historic racing. You can four-wheel-drift a 3.5 car (wherever it is safe to do so) under perfect control as long as you skill set is up to it. A modified 4.6 will produce so much torque that it becomes quite a handful, especially in the wet.
If you are determined to go down the T5 or similar route then I believe that John Eales does a bell housing to suit , but don't underestimate the amount of work involved in this conversion.
I apologise if I appear to sound a trifle negative about your plans, but the Moss Box cars are very special , you are lucky to have found a good one, so I would be strongly advising that you talk to many other owners before going down your proposed route.
