Welcome to TM Arjen I hope you enjoy your time here I have a 1999 +8 with the 3.9 engine and R380 gearbox with 31,000miles on both and it is a nice gearbox and suited to the torque of the V8. Not a gearbox to be rushed similar in that way to the Moss box.
Good luck with your project.
There is a lot of historical useful information on the site below.
http://www.gomog.com/allmorgan/allmorgan.htmlWishing you a Happy New Year.