I was pretty sure they were the same VK45DE engines. Looked it up on wikipedia to verify, and it is listed as VK45DE for both Y34 and Y50 bodies. The weird part though, is the Y34 model says it has 340hp, and the y50 model says it has 325hp. So....? Probably just tuning differences, as the engine in the FX45 models is listed as 320hp, and the President model still is called the VK45DE yet has a measly 276hp. If I look up the actual VK45DE engine it says it is a 340hp model and lists the engine as being used from 2003-2010 in M45s.
In looking up the VK45DE, I was quite surprised by its capability that was so underutilized. In race trim limited power spec it has 493hp, and non-limited race spec can reach 789hp!!!
ANYWAY. you can preobably buy a whole used engine and scavenge the necessary parts from it (or just use the engine as is?) for cheaper than aftermarket, new rods, bearings, and pistons.