[AZKen]

That's a very good question. When I installed my motor I didn't know. I just mounted the metal brackets to motor, lowered motor into position and added the rubber bits at that time. Pre assembly depends on which part of the rubber mount has the best tool access after touch down. You won't get into trouble because you can raise and low the motor while its chained on hoist. In my case, it is a 4X4. Different mount brackets. I just worry about shearing off the rubber by an out of control 900 lb motor/tranny.
Be sure, ahead of time, that the rubber motor mount studs/bolts used, will accept the washer and nuts thread on easily.

Even though I may think that pre-assembling the rubber to truck first will work, I still prefer to wait until motor is done swinging and jerking and sliding and close to final position before getting the rubber involved.

See what you think and do what seems best. BTW: is your tranny cross member removable or welded in place? I've seen both. Hope it's the removable type.

The motor will be slung with a downward angle to rear. It will "wind in" "rotate in". Have a jack ready to go under trans. To support, raise and lower and move around. Have the seat out and the trans cover off. You can sling the trans with a strap and move it around thru the trans hole if that's better for you. Have the cross member mounting, and the trans to cross member bolts/threads, pre tested and sorted before beginning. Shifter removed from trans. Dizzy cap removed. If front clip is off, it's a piece of cake. Did you replace rear output shaft seal on trans? If not, may spill oil upon tilting.
If I missed something or misspoke, maybe someone will correct/advise.