The tolerances between the tabs and the framing, and the tanks and the cleats were pretty close. Also, apparently my pre-installation of the cleats weren't all in the same plane. In order to get the tank fitted and laying flat on the cleats, I had to trim off bits from the cleats using a grinder and hammer/chisel. With the close tolerances, we had to gently lower the tank into the space with lifting straps, keeping the tank as level as possible to avoid getting caught up on the sides of the framing. After numerous cycles of install/test/remove/trim we had a tank fitted (with sore backs). The tank is secured to the framing using thru-bolts backed with tee nuts requiring only a single wrench to install/remove (no tapping screws!).
Port Fuel Tank Installation
As with the water tank, we had to trim some of the cleats to get a good fit, but it wasn't nearly as extensive/difficult as the water tank. Tank was secured to the framing using thru-bolts. The green fiberglass angle stock cross members were then bolted down (these act as a hold down for the tank, and as support for the floor).
With both tanks (and all tanks eventually) the lifting straps are being left IN, wrapped around the tank to help the next poor sod who has to lift them out (hopefully it is not me).