Welcome to the Forums and the world of '64 Overlander ownership!
The blackwater tank is, indeed fiberglass and is mounted below the toilet between the frame rails. There is a galvanized steel box that it rides in that is held to the frame with steel strapping (at least that was the case with my '64 Overlander).
The 12-volt fuses were monted behind an access panel on the old Univolt found in the streetside corner of the One-Stop-Service-Compartment (rear trunk). It has been a few years since my original Univolt was replaced, but I seem to remember that it was a rather large dark green box behind the battery/shorepower switch with the red reversed polarity light. If your original Univolt has been replaced -- the first place to look would be the replacement power converter -- to see if it has fuses behind a panel. Should that prove futile, you may be looking for a needle in a haystack.
Unless your coach's windows have been replaced, or were a custom order feature, they should be a standard Hehr model utilized in virtually all '64 Airstreams. The new replacement weather seals have a somewhat different appearance when compared to the original factory items -- they do seem to seal better, however. There are at least two sources for the necessary seals and gaskets - - - Vintage Trailer Supply
and Inland RV
The window crank gearboxes can be a bit more difficult to locate, although availability seems to be better today than it was nearly eight years ago when much of my coach's refurbishment was completed. You may be able to find these gearboxes at a local shop that supplies parts for older mobile homes (often found in larger college communities where I live). You can also check with Vintage Trailer Supply and Inland RV.
Good luck with your Overlander!