Howdy Tugg and All:
THe '68 block is a good one for a rebuild with forged steel connecting rods and a slightly thicker thin walled casting. Either block is a good candidate for rebuilding so long as the need to overbore is not excessive, say .030" or less. Once you get them disassembled and inspected you'll know. The fact that the E0xx block is stuck is not an indication, but getting it apart and assessing is the only way to tell for sure.
The '68 block should be drilled to accept both large and small bell housings. The OEM three speed trans in '68 Is a fully synchronized three speed called a "3.03". It is identified by its 9 bolt top cover. The 'E0xx block will likely only accept the later transmissions,like the 3.03.
On the heads, the D0xx casting is at the beginning of some positive upgrades yet to come between '69 and '75. The changes came randomly so this is an era makes it difficult to use words like "Always" and "never". The good news is it will have a 1.75" carb hole. The down side is that it will have 1.65" intake valves, NO hardened seats and likely a 1240 cc log volume.
So, if putting many miles on this engine is in the cards the E0xx casting is a better, less expensive rebuild bet. It will have a larger intake tract volume of 1,420 cc, 1.75" intake valves and hardened valve seats. Both heads have 62 cc combustion chambers.
You didn't mention what you have planned for exhaust, but FYI the '68 exhaust manifold is a good one if you plan to use stock type exhaust.
Keep it coming.
Adios, David