First of all ... the Dash on the picture it from a later W461 ... 290GD Turbo I bet ...
Second ... everything is possible ... if you have the money!!!!
These dashbords have a lot of electronic gimmicks older Gs like 1982-model don't have ... so if you really want to change it, you will have to do a lot of fine work. And at last ... I first would try to find out what the single dashboard would cost ... I bet than you wount change it anymore ... [;)]
By the way ... have you ever seen these dashboards live??? I have seen them and they look alien on the W461!!! I love the W463-dashboards and I like my W460-dashboard, but he latest W461-dashboard simply are looking poor ... [B)]
I agree with Arnd, the later 461-dashboards doesn't look troughly designed. It looks as they've fitted the parts they have available and just made them fit instead of re-designing the dashboard. Just look at the plate around the shifter gate too.
I bet they could have saved a whole lot of money if they had re-designed it, because it has to cost a lot to assemble that dashboard with all those screws, washers, clips and parts.
The problem with fitting such a tacho is that the signals are most likely electronic, even the speedometer. You'd propably have to change engine-temp sensor and fuel level sender too.