[bouncytruck]

I did some extensive research, including calling the Washington State Buckle-up organization and the Washington State Police regarding child seats for larger children in a vehicle that isn't equipped with shoulder belts as I did not want to do irreverable modifications to my truck. There is only one brand of car seat with a 5 point harness designed for children 40-80 pounds and this is the only one that works with lap only belts. It is expensive at $180. I believe I have a link on my home computer.

Outside of this one seat, it isn't recommended to use a booster for larger children. It is considered safer to belt them in properly with the lap belt.

I believe it was TonyP who build some ingenious billet brackets for rear shoulder harnesses into his family style cage. As Yukon pointed out, it may be best to tie the cage into the frame or, at the very least, reinforce the floor so it doesn't break free in an accident.