I found only two difference in the 250 and the BC400.
The big cam rear cam bearing journal is larger in diameter then the 250. This effects the location of one of the bell housing bolts. So to use the original 250 bell housing you must enlarge the one hole in the bell housing which is no big deal with a die grinder.
The front motor mount on the big cam is an OTR commercial truck type so you need to mod the frame to accept it.
I found all the dimension to be the same.
The extremely large big cam turbo causes a clearance problem with the 929's cab heater. But the high pitch whine of the turbo makes up for the lack of heat. I have not resolved this yet.
Just make sure you find a BC1, 2, or 3. The big cam 4 had a low flow cooling system that was a problem in these late model big cams.