The Dodge dealership can only retrofit top tether anchors (the "TCH" in the LATCH acronym which stands for Tethers for Children) in your van, not lower anchors (the "LA" in the LATCH acronym). The 48 lbs. child weight limit for LATCH in Dodge vehicles is set in stone in terms of the lower anchors for models that have factory equipped lower anchors, at which point it would become necessary to install the child restraint using the vehicle seatbelt, not the seatbelt and lower anchors, just one or the other installation method can be utilized, not both. It is however advisable to use the top tether at any weight with the lower anchors or with the seatbelt, and to continue using the top tether beyond the vehicle's specified LATCH weight limit. This is because the tether serves as a secondary point of attachment rather than the primary, and the tether serves a beneficial role by reducing head excursion, the movement of the child's head forward/beyond the protective carseat shell, during a crash. The LATCH manual has a detailed explanation of the top tethering recommendation at:
http://www.saferidenews.com/html/LATCH_P49.htm Your Britax Frontier also requires the use of the tether for kids weighing 65 lbs. and up in harnessed mode.
Your Dodge has seatbelts which lock at the latchplate by design when the seatbelt is buckled and you shouldn't need a locking clip. This site has a primer on seatbelt types and how they lock:
http://www.carseatsite.com/lockingclips.htm
If you find your vehicle's seatbelt slips instead of remaining locked at the latchplate, one tip mentioned in the above seatbelt primer would be to flip the latchplate over and then buckle the seatbelt to keep it locked.