I am happy to throw a curveball in here.
I have had nightmares with my toledo. It ca,e with std rear springs, and I think approx 160lb ish fronts (smaller diameter than dolly at the front)
I fitted the TR7 engine and fg front wings. The old springs still worked OK at the front. However, from purchase I would often have the rear bottoming out, just2 up (me at 11stone. daughter about 8-9 as she grew) and that was in undulating country roads. I tried nw tioledo springs. Same thing...
RBRR 2016 I panicked, and got some 1500 rear springs. Exactly the same rate as the toledo type, but 1" taller. I think 140lb from crude measurement. At the same time I fitted some nos sprint shocks and 175lb, 1" lower springs. Not a bad setup, but it was still bottoming out on the RBRR (I was joined by 2x 13stone blokes)
As RBRR approached this year, I satrted to worry. Co drivers were going to be much heavier
I got some ex JT rear spax/springs to try. The springs are I believe 175lb. The rebound adjusters actually freed off, but the ASP's are VERY seized. Luckily I had some spacers, and when fitted the rears gave a sensible height, and served us well on the RBRR. Since then I have found 1 or 2 up they are a tad skittish, and I really ought to knock the shocks back a few clicks.
But the issue seems to be the rear is problematic. Std too soft if carrying anything. Uprated may be too hard. However,this is a Toledo, so a Dolomite may have enough extra weight at the back to sort that?
What we really need are something that you can adjust the spring rate somehow. Those air shocks?
BTW I disagree with Steve about lowering the ASP's causes more preload? It can't. Moving teh ASP down will leave the spring longer,so less preload on the bench. Winding the ASP up will compress the spring more but unless you are compressing the spring 3-4" it will still be the weight of the car that is doing the real-life compressing.