If you don't have a block to rebuild, then you can buy a long block from several manufacures. Mine came from PAW. You can do a web search for ford crate engines and find many vendors. One I looked at the 289-302 blocks were about 900 and the 351W was about 950. The other parts you would need, like carb, headers, motor mounts, etc are about the same cost between the two engines.
Don't forget that there is a lot more to the swap than just the engine. Here is a write up of what is needed for a 65-66, the list for the 68 is similar:
http://www.geocities.com/MotorCity/3...ecial/6to8.htm
HTH
