Slant 6 to big block in 68 Cuda

Your 8 1/4 will be just fine for a street car. A well built 360 would be a lot less work and plenty fast. If your heart is set on a big block you may as well go with the 440. Schumacher creative services has conversion mounts and headers that are more than adequate for a street car. Not cheap though. You can build one he'll of a small block for the extra money in special parts to get the big block in it.