Did your boat not have side tanks before? Tanks in my Ultra through bolt from the bottom through the hull. I think the are 3/8" tapered and counter sunk so they're flush on the bottom. Not big on through bolting through the hull but other than glassing in mount plates, I don't think you have many options.
since my boat is super light and thin right there, i would think that the strap would stiffen the bottom up a little bit. My fear is that the bottom is so thin that with 15 gals of fuel hanging off the trailer on little pads will crack the bottom or start to. The bolsa checker board stops right around the dash area.The strap connecting the two pads together is unnecessary but it doesn't hurt. It does help keep things lined up while grassing the mounts to the floor.
This is exactly what I did except I didn't glass them in. As I stated earlier, the holes were already there through the hull. The tanks and mounts I bought didn't match the mounting points in the hull, so I made adapter plates so I didn't need the re drill anymore holes, I was able to use the existing ones this way. I then had them powder coated. Here's a pick of mount adapters and my transom platesIf you are crammed for space, and the wood block would be too tall, take a 1/4" or 3/8" aluminum plate, drill and tap it, and use countersunk bolts on the back side so studs stand up, then glass the aluminum straps to the floor. I have seen many done like this and always looks clean. My gulling already had carpet in it when I purchased it so I bolted tanks in through the bottom, took some stringer washers and countersunk them so I wouldn't be taking more glass out of the boat. Plus it gave me more area to seal off the holes. No leaks yet run floating, riding, or making a pass.