FORD 2010 E-450 v.3 - Manual (Page 125)

If you install a child seat with rigid LATCH attachments, and have
attached the top tether strap to the proper top tether anchor, do not
tighten the tether strap enough to lift the child seat off the vehicle seat
cushion when the child is seated in it. Keep the tether strap just snug
without lifting the front of the child seat. Keeping the child seat just
touching the vehicle seat gives the best protection in a severe crash.
Perform the following steps to install a child safety seat with tether
anchors:
Front passenger seating position
The tether can be attached directly
to the rear of the front seat.
1. Adjust the front right-hand
passenger seat full forward.
Seating and Safety Restraints
125
2010 Econoline (eco)
Owners Guide, 3rd Printing
USA (fus)