The T9 option is not going to be inexpensive. The M41, although now rare, will probably be much less expensive.
The other option to consider is a different rear axle. I know that some people have used the 9" Ford rear differential which was in production from 1957 - 1986. These have been used in Volvo Amazons. The 9" is 'semi desirable' because the differential is offset slightly on the rear axle. The 1972 and earlier 31 spline axle can be shortened. Save the short side and cut down the long side to match the short side and I think the over all width matches an Amazon within 1" or so. Some details on narrowing here
https://www.jegs.com/tech-articles/f...e-information/
Depending on the vintage of the rear axle you will end up with disk rear brakes which may or may not be desirable.
I don't know what the availability of the Ford 9" is like on your side of the Atlantic. Brand new old axles are available because the 9" is a popular option for North American hot rodders. Clearly this will require some fabrication so definitely more complicated than an M41 installation and probably about the same as a T9 installation. It may be cheaper than a T9, particularly if you have to purchase an already rebuilt T9.
If you do go with a Ford 9" rear end you will be set if you ever decide to drop a V8 engine in at some future date
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