If you think LATCH is safer... think of this. LATCH is only ok to use up to 40lbs (and in some vehicles 48lbs). The seatbelt is tested to withstand a crash with 380lbs. Now which do you think would be safer with a child that is too heavy for LATCH limits?
LATCH is NOT safer, it was invented to be easier for parents to install seats. There is no difference in safety between a properly LATCHED seat adn a properly belted seat.
Do not turn the seat into a booster, install it with the seatbelt, learn how to lock your seatbelt (READ YOUR VEHICLE MANUAL) and get a good install. If you cannot, find a CPST at www.safekids.org and an event in your area to help you install it properly adn show you how to do it. Also, always use the top tether of the car seat, regardless of weight. Top tether is necessary when using a larger seat like the Frontier.
For what its worth, I've never used LATCH to install. Britax LATCH clips are very nice and easy to use but hte seatbelt was easier for me and I preferred the install in every vehicle with seatbelt vs. LATCH. My 45lb 4.5yr old is in the Regent right now installed with the seatbelt and top tether.
Don't worry about the seatbelt failing, its not very common to happen, and if you use the top tether your seat wll still be in the car, and even if the seat IS ejected from seatbelt failure (again very uncommon), your child will be contained in the car seat and much safer than being thrown from a booster seat with nothing protecting him.
Again, learn how to install with hte seatbelt, and learn how to lock your seatbelts and get a tight install. If you need help, find a CPST in you area to help you.