In the case of AD trucks. The crossmember is multi function. 1) it supports the engine/trans. 2) provides support for the front leaf springs. 3) provides frame ridgity for the cab mounts.
In the case of doing a MII crossmember and drivetrain swap, you can erase all of the above. You're putting a crossmember in where there wasn't one before. Also, more than likely, adding a second crossmember just behind the original.
Take a sawzall, whack it out, never look back.
