Hey Matt, sounds like a fun project you've got going... VTS is a good source, I've bought a few tanks from them but never a black tank, bonus is that shipping from them is very reasonable. Having done a similar project more than once, I usually get my measurements and then shop e-bay. I've bought the same basic tank 3 times from the same outfit, I think madison wholesale. No frame mods but the drain hangs below the frame, I'm all right with that. I think the tanks are about 35 gal. At drain outlet 7" tall. Just did a '68 Overlander which I think is very similar to yours. I did not re-use the black tank but I thought the size and basic setup was quite good, you'll have to work pretty hard to make a much larger capacity set-up. What I almost always do is eliminate the black tank (we use a porta-potti) and put as large of a gray tank as we can get in that spot. On the '69 there should be the dropped rear frame section, I put it up in there with the drain through the lower part of the frame, that far back I don't think any real frame strength is lost. Drain valve just oust the frame rail and only below the frame maybe an inch right at the drain. Here's a pic of the tank, I'll try to find one of the install.
Many other considerations went into these decisions...
Good luck, Mark D