Now, could you slip the saddles in when the suspension is in the car, if you forgot to install them? Yes, for sure, you can if you have the 1" lowering spring. I don't know about trying to do it with the highly sprung oem spring. You might have to use a c-clamp or something to keep the spring somewhat compressed.
Another FYI: Each of the 2 bolts on each saddle, 4 total, has a weird shaped, sort of rectangular, washer at the top. This is to reinforce where the top of the bolt contacts the crossmember. Those 4 bolts and where they contact the crossmember are under enormous stress. I detected elongation of the holes in the saddles and the crossmember, due to movement of the various parts. This suspension had been monkeyed with before and was missing parts, incorrect washers and bolts were substituted, and one upper and one lower a-arm were badly bent, probably due to an accident or incident. There was substantial wear on the driver's side saddle, but not much at all on the passenger side one. One of the rectangular washers was missing, I had to make my own replacement.
Gosh, I hope that the crossmember isn't bent due to whatever incident occured. It shows no sign of damage, but it's really hard to tell if they are bent a little bit. My long time GT had a problem with pulling to the left, I replaced everything and had alignments, but nothing would fix it. The problem persisted for years. Under it's previous ownership, it had it's whole front end nuked by running under the crossbar at the back of a delivery truck. All the body panels from the windshield forward had been replaced with factory new ones. Finally, I had RallyBob send me a new crossmember(late 80's) and had my repair shop install it. Problem solved! But, looking at the old crossmember, it was impossible to detect anything wrong with it.