Strange S60 Rear End Help

Leave the perches loose and have them welded on once the car is put together. With leaf springs you can't guess since the leafs move around a ton. If you have a Calvert setup you might be able to mock it up but even then I'd wait until the car was together. If you have a four link you can figure out where it all needs to go before the car is built but leaf springs are tricky to figure out unless you have an identical car to measure.