Thank you Tony for all the infos!

My friend did upgrade his 72 Roadrunner 440 with the XHD leafsprings, 1.16Tbars, big swaybars front and rear and 17" wheels. That car handles great, but he/I don't know if it understeers or oversteers. With other words: it's not really calculated, he just installed stuff on it. But the car is very stiff, specially on the rear, those XHD leafsprings seem to be pretty stiff, but he seems to be very happy with it!

Our roads here are very good, heck, there is no 50miles of freeway here without a road construction because they renew the freeway every few years although it was in PERFECT shape! (politics: there is 1$ tax on each qrt of gaz that goes to road construction. Now you can make the math how much money they MUST spend every year to use the budget!!)
There are hardly big bumps on any road and nowhere a hole, so I can go from a car that won’t see bad bumpy roads.

Now, to the other points:

Tbars, your numbers are interesting. Firmfeels b-body numbers are the following:

diameter rate lbs/inch
.840 89
.920 128
.960 152
1.00 179
1.12 281
1.16 324
1.22 396

They have a different rating than yours, maybe because yours aren't for a b-body. Different length, different rating right? I would think that the rating changes with the LCA length, although the motion ratio is always 1.

But anyway, the Tbars are the smallest problem of all, the motion ratio = 1 so the wheel rate is the same as the advertised Tbar rating. That much I did understand

If I ignore/remove for once the swaybars and take once the 1.12" Tbar with 281lbs/inch. That gives me a total front wheel rate of 562lb/inch.
On the rear I measured the motion ratio of 0.9303. So the wheel rate is 0.9303^2*leafspring rate.
To get a 75% rate I need a rear wheel rate of 94lbs/inch PER side.
This means I need a leafspring static rate of 108lbs/inch! That's pretty low, isn't it?

The roll rate numbers you posted, I did calculate the motion ratio for those numbers and it's 0.853, I got 0.9303, that's much higher.

So just to make it easy, if I take a 120lbs/inch rear spring (higher than I should!) I can make it up by making the front swaybar bigger than the rear and that's usually the case.

But the most important: your swaybar numbers make me happy again! I can't use the numbers since it has a different arm length BUT they are all proportional when you ^4 them and divide them by the rate! The lbs/degree number I had wheren't!
That means: I will redo the swaybar test today again but this time with the other wheel blocked so it can't move up.
Then I will have the rating for a b-body front swaybar with it's specific arm length and 1" diameter. Then I could calculate/average the rating for the firmfeel 1 1/4" or 1 1/8" swaybar IF it's not a tube, what I don't expect.

Same I can do for the rear swaybar. I will call them and ask what arm length it has, from this I probably could calculate the 3/4" swaybar. I won't get accurate numbers but it should be enough I hope!

Interesting topic here, it's really fun to do more than just bolt on! Now it hopefully also works!