V8 Question !!!

Please could somebody tell the physical differences between the 3.9 V8 motor and the later 4.0 V8 - partically the block (short engine). I wanted to know if anybody with a Series 2 Disco V8 (4.0 litre thor engine) has ever used a 3.9 block to do a rebuild with. As you have probabley guessed i have a terminal 4.0 with a cracked block - the car is at the age (1999) where throwing £2,500 - £3,000 at it for a recon motor isn't viable, i was looking into the feasability of using the cheaper more readily available 3.9 unit and creating a hybrid to get me out of trouble and give me a few more years motoring out of the old disco. Any thoughts or advice greatly received. Rooster