Spring in first, then the FP, and hold the FP forward/inward while you drive the cross pin back in.
The cross pin needs to go on the back side of the rim of the FP to retain the FP in place.
Hence the FP cup is channel drilled, so the cross pin not only holds the cup in place, but the FP in it as well.
If the cross pin channel is loose of the pin, then once you get the pin in place, force it to one side enough (while still retaining the FP) that you can add some Red loctite, push it to to the other side to protrude the same amount, add some red loctite (272) to that side of the pin, push it even in the receiver, and give the loctite 24hrs to set up to just hold the pin in place.
Note, don't go buck wild with the loctite to glue it solid in place, just the ends of the cross pin to hold it tightly to the receiver instead.