Love to see more FS Frankebikes. Though the shocks on mine are a joke, it's worked amazingly well for me.
Interesting shock mount bracket. Is that a repurposed caster? If you have not tuned it yet, this is how I did mine. I took the original frame and fork, and put wheels on it. Then I measured the height of the pedal crank from the ground. Then when I put on the rear assembly, I made sure the crank height was identical. This makes sure that you haven't ended up with a funny fork angle. Mine ended up riding perfect, with no funny handling issues.
But I do think you must center that swingarm in the frame. Id take a hacksaw to the long part of the BB, or find a cheap bb that is equal in length to install on the swingarm.
Love your bolt on shock mount. Much nicer that what I did, retaining a large chunk of the second frame.
Once I was happy with the connection, I welded my bb to the frames rear droputs. I didn't like relying on just bolts at that point. You can only do that if the front frame is steel of course.
But if you bolt it, which should work fine really, I'd think that some kind of torque arm like device might be good. What I mean is a plate with a round hole for the bottom brackets "axle" to pass through. This then attaches to the front frame. It's not to prevent rotation like a TA, but just to prevent a complete separation of the two parts of the frame. That way if your bolts slip, you get some chance of stopping and getting off before the rear wheel goes bye bye.
Most of the riding forces just want to make the bb jam into the dropout, but you might still have forces when you jump off a curb or whatever going out of the dropout.