It's the rear spring cups you need to lower it. You can use a full coil over on the rear as long as it has a 1.9" id spring, you'll need to strengthen the mounting point on the trailing arm too. You could use a non coil over damper as long as you have the adjustable spring seat converters. I've got a set of ohlins coil overs in my garage. £4.2k to buy new, £3k second hand, a bit too much for a road car though.
You can get the seat converters from Gaz and KW from memory. Then use your choice of spring with a helper spring and divider.