Well, you CAN get adapters a lot cheaper than $500. Most of them are crap
. This is NOT an area to skimp and get "cheap" on........when that cheap adapter breaks, causing the wheel to fly off the car and PASS YOU ON THE FREEWAY....you'll wish you got the good adapters
Good adapters are available from, among others, Motorsport Tech outta LA
Motorsport Technology Wheel Adapters
If it is a 90 T-bird, it PROBABLY has the 5x4.5" Ford bolt pattern. Some GM is 5x4.75", and OUR cars have 5x5".
In addition, measure the backspacing (write back here if you don't know how, and I'll walk ya thru it). You want to end up with approx 4.5" of backspacing on those wheels.......BUT if you need to do adapters (probably) then you'll need at least 1" (and probably more) of "extra" space to accomodate the adapters.
That would mean that the WHEEL would need (for example) to have 6.0" of backspacing, and then you use a bolt pattern adapter that is 1.5" thick to get the bolt pattern right AND get your backspacing right.