Rule of thumb on core engines is $1.00 for every cubic inch. $440.00 dollars would not be bad for a good core. $1500.00 for a nice rebuild if you can do the pulling and setting of the motor yourself. Also depends if you need pistons and an overbore. But if you buy a new rebuild kit that comes with all the gaskets, bearings and seals, cam kit and new water pump, then its cheaper. The amount of machine work will be a factor. Over bore and you need new pistons. But if the motor goes back standard with just a hone then it is less expensive. you will have to open the motor up and see what you have. Let us know what you decide and if we can help. PS the 68 motor is pretty good, forged crank, ect.