Manny,
We met May, 2002 in Daytona, garaged next to Marcello...Blue ITB from Buffalo, N.Y.
One solution we researched is the light weight stock car wheels, but they are only available in 13 inch sizes, with multiple offsets and widths available. I bought them back then, and the offset easily cleared the brake calipers. I have not seen another really inexpensive, and useable rim for this car. Check Weld, and others like it, that cater to the Mini-stock racing crowd.
Incidentally, ITB Mustangs are not "limited" to 15 inch rims. We got them years ago when it was demonstrated to SCCA that the '90's cars were produced with that size. 13's were used in the early years of the listed "line" years (1979-1993).
Good racing.
Bill