OK, I had a chance to look at this in more detail. I've recently been shock shopping, and ended up with Hagon's, which are expensive but can be customized to your needs, and I highly recommend. Ikon brand can do the same.
One of the Honda/Showa OEM shock assemblies (52400-447-733) listed for the 81 CM400T is the same as for my 83 CM450. I can't speak for the other ones listed, but here is what I know.
My OEM shock has a Center to Center length of 320 or 321mm, and the stroke length is "over 3 inches" or "more than 77mm."
If you choose a shorter CtC shock, your rear seat will sit a little lower, and you may be able to give up some of that stroke length safely.
If you choose a longer CtC shock, your rear seat will sit a little higher (at least, when not laden with the rider). If the spring rate and/or preload is high enough, you will add some compression but loose some extension. But if the shock has the stock or less stroke length, you may lose a lot of extension, which is not safe. It may not keep the tire on the road coming off of bumps, or on a sudden declivity.
Besides Eye-to-Clevis, Center to Center and Stroke Length, the other factors are Damping, and Spring Rate (and/or Progressive Rates). There is also number of preload stops. The number of preload settings becomes less important if you have a custom spring.
Since I "adventure" on gravel a lot, I got a Hagon 2820 330mm CtC with 80mm (3.15 inch) stroke length. It is custom sprung to my load specs (my weight, passenger weight, gear weight, how often I ride 2-up versus 1-up) with a 4-level or continuous progressive spring. The progressive spring allows for a plush ride over minor bumps while retaining firm control on big bumps.
The Hagon has 20 Damping settings, and the rebound damping is linked to the compression damping. This is a selling point over the Ikon, where one is adjustable and the other not. However I have Ikon's on another Honda, my Valkyrie, and it is a high quality shock that solved the problem of constantly bottoming out.
Before I got the Hagon's I had some $99 shocks, the Common Motor Heavy Duty. They did their job, performance handling was very good, but the ride was unnecessarily harsh. If you weigh 300-400 pounds, it might work better for you!
