I would recommend waiting until the display arrives. Maybe offer the guys $30 or $40 a piece to help you mount the set. It would be a shame to install the mount first and find out you were off.
It doesn't take much more than 10-15 minutes in install the mount and hang the display. Have the guys hold the set up so you can gauge the approximate location, then install your wall plate in the correct location, install the rails on the back of the set, hook up your cables and your good to go.
Really, you're only having the guys hold up the set to gauge your position for the wall mount and then having them put it on the wall mount once the rails are on.
Or if not, don't you have a nearby friend that can help mount the set?
Klipsch RF-52 II // RC-52 II // RS-42 II
Klipsch RW-12d // R-110SW
Klipsch KG 2.5