A lot of the real world usability of a hardtail depends on the weight of the bike and the rider and as others have mentioned, whether the seat is sprung or not.

I have built a couple of mid-sixties Triumphs that I converted to hardtails and with a small engine and a decent sprung seat, they ride just fine - not plush like a softail, but pretty damn good and you can put plenty of miles on them. One of the guys I ride with converted a 96B Crossbones to a hardtail himself, and I've seen him hit some pretty bad bumps - he swears it's no big deal but once you see his *** slam down then come a few inches off the seat - there's no question he's just in denial.

All that being said, those old trumpets weigh about 350 lbs wet, and the average softail is closer to 700 lbs so every time you hit a bump, you're driving twice as much weight into it.

Personally, I'd avoid the combo of a big, heavy bike and a rigid rear end unless you live in an area with great roads or just plan to bar hop.

 

I think I agree with that, you need something really minimal
If you want to Hardtail a Softy, you need to lose at least 100 Pounds, losing the suspension/swing arm will give you 55-60, you need smaller sheet metal, no dash, smaller bars, as much aluminium as you can (specially wheels), maybe even some of that advanced FibreGlass for the fenders.

 

I had a hard tail triumph in 1979 ,that I built in the dining room , really looked great and I was in great shape! But I remember cringing Everytime I saw a bump or joint in the highway. I sold the bike after awhile because it hurt! I remember when my first air ride in my softail blew out and I had to ride home on the bump stops. I thought my teeth were going to fall out.
Got the shot gun shock now and all is good , you can have both worlds with it.

 

My 1st bike was a custom hardtail that I still have with a 250 rear. My second bike was my deluxe. But it is a rigid frame. I have tried many things such as spring seats, cchoppershox, etc.... but by far the best and current setup I have is goodyear air bag underneath my seat. It absorbs some rough bumps and a lot better on my back

 

your letting youself into a world of lifetime lower back problems and pain. Been there and it's taken the fun out of running rigid. Even with the 91 swoftail I'm a bit rigid. Serious back trauma happens cumulatively not always all at once. Look at what the geritol generation is riding these days, 3 wheelers and scoots with more accessories than I thought was possible.. HD have turned into 2-3 wheeled motorhomes. I'm embarassed at times. I went down hard 14 months ago and rebuilding the totaled scooter from my wheel chair was therapy for me. Never once considered never riden again. Thgo it is more dangerous on the rouds these days. I got plowed by a unlicensed cell phone yaking punk who was looking eye to eye when he cranked his wheel anyways. So much for multi-tasking.

 

I rode a couple of my old kz400 and 650s rigid. I had both setup with a solo spring seat. It was comfortable enough for my 18-22 year old body, but I have no desire to go back to that. It was just for the sake of a clean, cheap, scrappy, cheap and mostly cheap bobber rat style build I was into back then.
I agree with what others here have said. If you are gonna do it, cut the frame and do it right...