Just had a thought. Assemble the spring ,shock,wishbones off the car and compress the spring either with a strap or spring compressors and then torque up.
If I can find the correct length of the spring when on the ground that should sort it.
Heath Robinson eat your heart out!
John the reason the weight has to be on the suspension is not to compress the spring, rather to seat the bushes in the bottom arm before you compress them by tightening up the bolts.
When I did my front subframe it was easy enough to torque the bolt for which you have an access hole at the front of the subframe, but as you can't get a socket on the rear 2 I just used a spanner. Since then CJJ came up with his excellent guide and crows foot idea. The crows foot sets are not expensive on flea bay