1988 740 Rear Suspension

Most people convert the other way around, I've seen tutorials out there so it shouldn't be too hard to figure out what you'd need.

Have you looked into aftermarket overload springs?

Cars with the Nivomat (self levelling) system use lighter springs that cars with the conventional shocks, and the Nivomat shock absorbers provide some of the springing function. However I've seen people just add the Nivomat shocks to the normal spring system and be happy with the results. So all you really need are the (expensive) Nivomat shocks, but to do it "right" you should change the springs too. Personally, if I was in your shoes, I'd get the shocks, then analyse whether the springs also need to be replaced after testing.

Pay the money for Nivomats and fit them (£100-ish each side, last time I looked). Quick and dead easy job, but like all Volvo jobs you will need a 14mm socket.