Welcome from Colorado and to the vintage Airstream hobby. A 67 Overlander is a very worthy renovation project. They were well built in my view. I renovated a 66 Trade Wind 24', one year older and 2' shorter than yours. However, there was a major design change in 67 and you have the redesigned bath which is much nicer than the one in my trailer.
black tank was woefully inadequate. It was only 3" deep. That old clap trap toilet was also a nightmare. And my bath floor was also rotted out. So I rebuilt the bath.
Plastic RV waste water tanks are readily available. Inca Plastics in California is the source I use. I don't know the Canadian market. So as mrdes8 said, make your design and then shop for a tank that will fit. Guess what, the toilet needs to be over the black tank, and there is little room under the bath floor for a adequate black tank. Or you can use a bucket with kitty litter in it. I know nothing about composting toilets but I do like flush toilets.
Here is a photo of the two new tanks from Inca Plastics that I installed in my fromer Trade Wind. I moved the new toilet forward and the new black tank in the next to rear frame bay. I also drained my new tanks under the frame rail on the port side kinda like newer Airstreams. That way I could install a larger tank. And here is a photo of the resultant bath layout. At least it functioned much better than the old one.
You can go to the Forums Knowledge Base at the bottom of the Forums page to learn all about your 67 Overlander 26'. Click on Overlander and click on your year range. Then start reading what others have done. You might enjoy starting a project thread so others can follow along with your project one step at a time.
WBCCI #8607 VAC Region 11
KnowledgeBase trailer renovation threads: 69 Globetrotter, 76 Sovereign, 75 Overlander, 66 Trade Wind Such fun !