Can you spell degenerative disc disease? I can...
I usually keep my opinions to myself when someone talks about putting an A65 engine in a hardtail frame, but my back is particularly sore tonight, so I feel compelled to say "Don't do it; you'll regret it some day".
If you have to do it though, here are a few suggestions: 1. A springer works better with a rigid frame than a telescopic front end. 2. Most manufactured hardtail sections lengthen the frame by six inches and lower the rear by four inches. If you don't shorten the front end, it will throw off your steering geometry. 3. Note that early HD and Indian rigids placed the rider and foot controls well forward of the rear wheel, allowing the rider to take more advantage of the front suspension. Also, many seats were mounted such that the pivot point was well forward of the saddle, such that the entire saddle could move up and down, and not just the back of it. (Of course the ultimate in seat suspension was the "pogo stick" seat, where the entire seat could move straight up and down on a sprung vertical post.) With a Brit bike, the rider is very close to being right over the rear wheel, so if you're going to ride it on anything but the smoothest boulevards, it's a good idea to fit it with a 15 or 16-inch rear wheel with a big fat tire; this adds more than you might think in cushioning. When designing the seat, get the pivot point as far forward as possible on the frame, and install the longest springs you can manage, preferably progressive.
FWIW, the bobber in the picture looks pretty effective, except for the low profile rear tire and no-frills seat.
And remember, even if you manage to set it up such that the ride is bearable, every time you hit a bump, the entire bike is taking that shock, not just the rear wheel and swingarm.