The big issue with the older 6VD1 and 6VE1 is they only used 2 or 3 oil drain holes in the pistons and they were smaller then on most engines so they plugged easily. The upgraded pistons which you should get with a rebuild have 6 holes in them. This is according to Jerry Lemond.

Also according to him, the 3.5 DI cannot be swapped for the non DI engines. It is a completely different animal.