I would say its one of the best options for J40 diesel conversion. Have not heard that anyone has done this in Pakistan, but yes, many successful conversions have been done in US and Australia. In Pakistan, the one case I know is of @hsumbal who has transplanted 1HZ in his FJ40. And as per my understanding, it is a great success for him. 1HZ is a successor of 2H engine and both are of Indirect Injection mechanism where as 12HT is of Direct Injection mechanism with more horse power, torque and with turbo charging.
I would prefer 2H or 12HT on 2B or 3B diesel engines. Although, B series engines are tough as nail, but they are extremely noisy engines. Whereas, H series 6 cylinder diesel engines are much smoother and equally reliable. I have driven both series. And I will vote for H series.
2H is easily available in Pakistani markets as it was the standard engine of HJ60s. As per my knowledge, 12HT is not easily available and also very expensive. 13BT is also another option for such conversions which is a 4 cylinder, direct injection diesel engine. As for transmission, I think H55F transmission could go with B series and 2H/12HT engines.
Regarding weight distribution for 6 cylinder diesel engine on 40 series LC, its true that it will become nose heavy and you will have to counter that by upgrading front shock absorbers and leaf springs. But details comments could be asked from hsumbal.