The basic question to be answered here, is: "find an original dashboard and seats, or modify completely?"
There are obvious merits and demerits to staying original.
Pros
Maintains a "period" look that goes with the body styling.
Perfect fit, without need for any jugaar.
Simple, unadorned layout.
Cons
Misses out on some important creature comforts (A/C, various switches for different functions), and important engine sensors (RPM, gear chosen, warning lights, etc)
Difficult to find in pristine condition.
Difficult to fabricate according to original specs.
Ergonomically crude.
Original seats are not only very basic, they lack lumbar support, adequate "bucketing" and side support, and will never appear to be truly "beautiful".
The vehicle is going to be modified to be RHD. Finding a RHD dashboard is going to be even MORE difficult than finding a LHD version.
Verdict
In any case, the original seats have to go. They're just too basic, and uncomfortable...in addition to being downright ugly. I love the seats in my Range Rover. Leather, and electronically adjustable. Will have to look into the potential to install such seats, front and rear.
What contemporary dashboard can be used to replace the original? Lexus? Crown? Camry? A late American model dashboard by Chevy, perhaps? Must make it a point to visit some chop-shops and kabaar-khanas to investigate the possibilities. Of course, conforming to measurements is going to be all important.
Another option could be fabrication of a dashboard in RHD, fitted with the right sensors, as per need. This is possible, albeit time-consuming, and not very cost-effective. Need to consult the interior design Ustaad, on this aspect.