Ok... if I ran into this problem someone else might. I figure I had to go through the pain, why should anyone else!
If you have a 68 Impala that you want to drop a Big Block into, there are several problem areas that I have found. First, the mounts. The mounts for those cars come in two styles, the "Short & Wide" and the "Tall and Narrow". To find out which ones you need, measure the tops of the towers. The widths are either 2 3/8" or 2 5/8". The heights are much different, either 1 3/4" or 2 3/16". If your car has the original engine in it, check out the width on the engine mounts before purchasing. I bought the interlocking style and they are too tall. Once the left side was in place, I had about an inch that the right side needed to sink to put the pin in. Way too much, they were the wrong height. Check out Energy Suspension for the right mounts:
Second, the position of the motor is tricky. Should you buy an after market oil pan, be careful. Yes, the manufacturer will tell you that it fits the car, but in reality it is extremely close to the steering arm. In my case the oil pan said it would fit, and it does... kind of. There is room, but not enough for me to feel comfortable with. I am going to move the engine and tranny 1/4" back. This will require either purchasing a new set of towers and drilling new holes 1/4" forward, or using the ones I have. Either way, in my situation I need another 1/4".
If any one has a set of towers for a 68, please let me know. I would like to purchase them and fabricate them for my needs. The same towers were used for small blocks and big blocks.
If you have any other questions about this swap, let me know. Thanks!