So we ended up using external spring compressors and binding the springs with ratchet straps. Had a heavy guy stand on the crossmember while we jacked up the car, secured the spindle and cut the binds.
This is a very sketchy way of doing it, but the spring compressors simply could not compress the spring enough to be installed.
Everything else pretty much went right together. Now both front and rear have upgraded brakes and 5x4 1/2 wheels.
The ride height is even right now front and rear, but you can see the springs are slightly bowed due to the spring rate. We'll see how it settles with the weight of the motor, we can remove and cut the springs again if necessary and the rear ride height can be set with the air shocks.