Not being funny Freddie, but he may have actually downgraded it from the 4.2 to 3.9... :doh: Have you not got any paperwork?
The 4.6 is usually coil packed due to the later GEMS distributorless engine management, and the 4.6 is a crossbolted strengthened block. The 4.6 is a serpentine engine with one long poly belt running all the ancillaries. The oil pump on the front is different as well.
It is possible to change timing cases and run 4.6 on distributor-engine number should confirm what it is