It would be good to have a picture from the other side of the heater to get a better idea of sizes. It looks like the return pipe from the heater is below the intake pipe, and after it snakes left, it angles up at 45 toward the return manifold.
So I'm seeing two basic possibilities, and you can only pick one with careful measurements and planning. Make some sketches.
1. Change the heater return to angle up at 90 rather than 45, then 90 again to the front so it's a straight shot to the manifold. See if the SWE fits in that horizontal straight shot. Pay attention to the installation manual where it says how much of a straight piece of pipe must be before the cell and/or sensors. That's important.
2. Same as above, but put the cell in the vertical section. Then it looks like the outlet of the cell will be too high to make a 90 into the manifold, so you'll need to make some kind of bend back down in that shot. Either three 90s (front, down, front) or maybe a 90 and two 45s.
Pay attention to the installation manual re cell and sensor postion. Getting it wrong can fake out the sensors or even allow hydrogen gas bubbles to hang around in the cell. We know how that went with the Hindenburg.