Question for you that have built a kit. What is the measurement from the AN400's rear axle centerline to the back of the seat(as in the plans)?
The 650 has an attached rear swingarm that must be used, unless you re-engineer one. From the axle centerline to the very front of the cylinder head its 49.5 inches long. My thoughts are to place the engine up close to the seat back, after cutting the bottom tube, and weld in some tabs for the two engine mounting bolts. I do not think the rear axle would be much farther back, maybe a couple of inches. That cut bottom tube would be replaced with one going higher over the top of the engine.
Here is a video of a 650 engine being removed, just so you can see its size.
Here it is... 35 inches. Here it is why it is 35 inches. My seat is a semi-recumbent. If it were upright it would still be around 37 inches and that is absolutely at the back. My engine firewall is at the 31 - 32 inch mark. Unless you intend to heat up your back side on the cylinder head(s) you will need the firewall with possibly some heat barrier material on the engine side of it. You really need some air between the heads of the engine and the back of the seat. You are realistically 14-1/2" short and that is with the engine touching the back of my seat. Add another 4 to 5 inches and you are almost 20 inches shy.