Hey Chris;
I am in the midst of building a sbc with the e-tech heads as we speak. I am using a 89 block which has the one piece rear main seal. However you can use those heads on the two piece rear seal blocks as well, without any problems.

If you want to install the E-tech heads, which are the ones I would recommend along with the intake Edelbrock recommends, they will bolt right in. If you want to use a hydraulic roller cam on the 1968 block, you will need to buy Comp Cam’s roller cam for the earlier style block and the 850-16 roller lifters or if you decide to go with a solid roller cam, you will need their solid roller lifters #818-16 or 888-16. Along with that, the recommended push rods and a true roller timing set for the earlier block, and you are off to the races.