I did this on a fresh subframe right after reinstalling it to the body like you have. A coil spring compressor was a must.
The "internal" type which is basically 1 big piece of all thread is what you will be looking for. I "borrowed" one from AutoZone.
You also have to be careful when compressing the spring so that it does not start bending to one side. I found that setting the fingers top and bottom 90 degrees from each other helped. Also make sure both ends of the spring under compression are not pointed at anything (unless it's the neighbor's house :))
If you run into another problem I had, which was abused compressors (broken) from AutoZone, Rick's Camaro's sells a new design especially set up for our cars.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI
