Ford Repair: tires 1984 gt350 anniversary model conver, anniversary model, gt350

Question
I may be buying a 1984 gt 350 and the dealer says the rims are metric and therefore need metric tires.  Was this the standard for the convertible?  Where to buy these tires? Can I switch rims to avoid the metric problem?
Thanks

Answer
the tires are an odd size, 390mm, or about 15.3 inches.  The only tire you can purchase for them is the Mitchlen TRX tire and it is very expensive.  They were 130 dollars a piece back in 1984 and there still 130 dollars a piece.  I would switch to a standard 15 inch wheel but dont get rid of the rims and if you can afford it get the tires because they are kinda rare and a good gt350 should have its original rims.