I don't know what Corvette salvage yards you talked to, but if they indicated that you could convert 68 doors to work like a 69, I wouldn't be going back to them. I'm not saying it can't be done, but it's not a simple bolt in either.

To start with the 68 door skin is different because of the push button release instead of a key lock, and the outside door handle itself is different. Not only does the outside handle operate differently, but it mounts differently (one center stud instead of two evenly spaced studs) and it's shaped slightly differently. To mount the 69 handles in a 68 door would require modifying the inside of the door to install the later handle, but you might also have to modify the opening for the outside handle (the 68 casting is different enough that it uses a different handle gasket then 69-82 handles).

I'm not positive, but I don't think you could leave the push button in the door, to use as a door lock either. To start with the 68 latch mechanism is 68 only, the levers on it are different and so is the opening rod from the push button to the mechanism, compared to the 69 up handle opening rod. Since the outside key lock locks the door by a rod going to the inside handle/lock mechanism, you might be able to leave the push buttons and just use the lock portion, but if not then it would necessitate glassing up the push button holes so you could install a lock.

The only thing that 68 doors have in common with 69 up doors is the inside lock/release mechanism the inside handle and lock **** attach too, the 3 long lock/release rods and window regulator. Everything else, the outside handle, lock/push button release, release rod, latch mechanism, door skin, door inner steel structure, the glass, window tracks, and inner and outer door glass seals/wipes, are all unique to the 68 door.