MKII Corty 5 speed and propshaft conversion

Gents, (lady's too) good afternoon.
 
Let me introduce myself, my name is Frans living in the netherlands.
The car, a Cortina MKII GT.
 
In my younger days (I was 23 and just married) I bought this car for daily use begin 1979. The car was 9 years old by then and not worth much. On the other hand, the first owner treated the car very gently but since he was an old guy he messedup the clutch. So doing a new clutch was my first project. Altough I was a tv repairman this was easely done on the street.
 
We started a family and enjoyed many vacations to the the French Alps with the Cortina untill 2004. From 1987 I started to use the car only for vacation and let a golf diesel do the daily job in normal life. At this moment the Cortina is resting for 17 years what is really pissing me off. Finally (I'm 64 years now) I decided to make it roadproof again and need to do a lot of work.
 
To make it acceptable to drive these modern days is to get rid off the ever noisy propshaft. arround 50miles it start to vibrate, something we accepted in the past but now it is unacceptable. The plan is to make a split shaft with a bearing in the middle like the sierra. Another plan is to install a 5 speed gearbox and that is my guestion for now.
 
Does anybody now what possibillity's there are? Any input is welcome.

Hi Len, thanks for youre reply. To clarify the questions, It's a kent crossflow 1600GT. Spaghetti exhaust and double Weber followed by a manual 4 speed and 3.9 diff. During my caravan period I used a 4.1 diff but at this moment I installed the original 3.9 again.
 
The propshaft:
Like a said, it's an ongoing issue from the beginning. I bought the car in '79 at 50k miles on the clock and it was never misused. I heard from specialists in the netherlands that some of the models had this problem from new. The only solution apears to be a split shaft with bearing half way the tunnel.
 
For decades I have been listening to the advise of many enthousiast and till now I tried to solve it with a rebuild and ballanced replacement (tree times). Tried another gearbox, replaced all the mountings, new leavesprings, completely overhauled my own gearbox (especially the bronce bushing in the tail) All the rubber in the rearaxle support, engine mounts, etc etc.
 
I have a complete Sierra propshaft that needs to be rebuild ofcourse for correct length. Good time to work on a 5 speed. I have seen one once locally and I recall there was a gap of more that one inch between the block and the bellhouse. Meaby someone knows if there is a handy fit available.