i have seen these adapters in Truckin' magazine for sometime now. i have even looked for a 6 to 5 lug adapter just for fun. they have so many pictured. i was just curious to see if i could find one. well, i never did. i am sure they could make one though. as far as how safe they are i could not tell you. i have never had a set. nor do i know anyone who has.
the first thing that comes to mind when i think of this idea is the additional rotational weight the wheels and tires will add to the truck. this is a lot of stress on the OEM brakes. an upgrade would be needed to prevent premature failure of the brake system. by adding adapters to the scenario, along with mounting hardware, this stress on the OEM braking system would increase. i would definitely say it would need to have upgraded brakes at the very least. as far as the adapters themselves go... as long as they are made with good workmanship the adapters should be okay to use.
if someone knows any better please jump in. i would think it would be a good idea to sell the wheels you have to get money towards the proper pattern for the new AV. just my $.02.