I have a +8 with a T-5 transmission. I have adapted the T-5 to a 3.5 engine and recently changed to a 3.9. I also live in the U.S. and have access to D&D Fabrications who engineered this for Me. Mr. Eale in the UK would probably be a better choice for You. D&D supplied the bellhousing, internal clutch slave, and a GM Clutch as well as the T-5 transmission.
I agree with the others. You are ruining a good setup. The Moss Box has its limitations, but is IMO a delightful transmission. Some say You need a 5th gear. Well I rarely use the 5th gear on My T-5. I use 4th as I like the power. On long trips I will use 5th on what You call the "Motorway". But I avoid the Motorway's when I drive My Morgans.
If You must have a 5 spd than a R380 is the way to go for ease of converting. In anyevent You will have to cut and reinforce your chassis to accommodate the bell housing. As was pointed out You will have to remove the clutch mechanism and use a hydraulic setup. Nothing is particularly hard to do, but there is quite a lot involved. On My 3.9 I was required to cut the back of the crank shaft 1/2". Did it with a Makita Angle Grinder (cut off). I am an accountant. I did it OK, BUT! looks like Hell. How did I know to cut it off 1/2"? Mark at D&D instructed Me and sent a drawing. I was lucky that I did not ruin the engine.
Contact Me off line if You want more details. But My advice is don't do it. Also don't go to a 4.6. If You have not driven a +8 much, I am quite sure You can scare the P-- out of yourself with a 3.5, If Not a little modification will give You sufficient power to do so.
BTW: I have a spare Morgan/Rover R380. However: I live in Seattle. But then Ships go by My house going to Europe. Maybe You have the connections to make this happen.
