There are quite a few differences between the two. I do not have much time inside a SOHC 12V engine but Frank has been heavy into his SOHC project and has the DOHC heads sitting there. Seems to find the oil passages to be different.
The 3.0 DOHC engines do not draw much money from the importers and can cost less than a proper rebuild of an existing engine. Obviously the engine support is much different and will need to be swapped. As in wiring, water hoses etc.
There has not been a production RWD 3.0 DOHC so the proper water outlets will need to be fabricated.
I am going through this now on a car I am building.