The calipers changed when the steering damper blades were discontinued. Later calipers don't fit cars with steering dampers apparently.
On the matter of weak rear springs, one solution would be to buy a single new spring. Strip it down, and add one extra leaf to each side. You wouldn't need to touch the main spring fasteners at all, just remove the nuts and lower plate on the U bolts. Lift the axle. Take the nut off the centre spring bolt. Maybe mess with the spring keeps at each end a bit. Insert the extra leaf, and bang it all back together. There is sufficient threads on the U bolts to do this.
I once did this to an MGB.........when we were loaded with daughter and luggage.
2005 Corsa Red Roadster S1
2016 Saffron Yellow (Narrow) Plus 4