HI, im in the middle of this swap right now. In the UK there are certain legalities where cross members shouldnt be removed. Since i am sticking to "the rules" i have had to make up a small transmission tunnel on the cab floor to give enough clearance for the trans over the cross members. So i would say if you dont want a trans tunnel then you will have to remove/replace/modify the cross members.
I have also moved the engine over an inch to help with header/steering box clearance - it will still be tight, and i plan on making the headers to fit.
Hope this helps.