Yes there is a seat that will offer a 5pt harness to 80lbs. It is called the Britax Husky. You can find it on Britax's website at www.britaxusa.com . There is many places online that also sell them. The seat does cost about $200 though, but a Britax is WELL worth every single penny! :o)
Jen E aka carseatmom... Mom to 3 and a Cert. Child Passenger Safety Tech!