I dunno, I don't think I have to. I mean, the 2.8 and 3.4 blocks are basically the same. The only real difference is the intake and TBI. I will have a bigger clutch installed and a winch up front adding weight but that's it. None of the other guys who have done this swap have mentioned adding spacers.
I still have a lot of other little mods to do once the motor is in and working correctly. I can't wait!
Bart