[bret]

Ask lots of questions:

Steering radius loss...the manufacturer should have turning angle data
Bumpsteer...get actual bumpsteer graphs from the manufacturer
Bracket flex...your trying to twist big tires on a heavy car
Strength of the rack itself...many of these racks were Originally designed for much lighter cars with much smaller tires

It is extremely tough to get a proper rack in the proper location on a car that was not originally designed to have one.