I think I'm good. I just looked up the parts in the Timken catalog.
The bearings for mustangs with 302 and 351 engines from 1968 to 1995 have a bore of .671". These part numbers are FC65174 and FC65662.
The 200 6cyl from 1980 to 1982 uses FC65174. 200 and 250 6cyls from 1973 to 1968 use FC65662.
The 4 cyl, 140ci Mustangs use a bearing which is .590"
1974 to 1979 171 6cyls use a bearing with a .593" bore. The part number is F33126.
The bearing for 1955 to 1957 T-birds has a bore of .669" Part number 203S. The same bearing is used for many 1958 -68 with 233, 240, and 292 engines.
Bill's 6203 bearing is 17 x 40 x 12 which works out to about a .669" bore
Based on this info, it looks like you might be using a "light duty" T5. It also explains why John says the stock bearing can be reused.
Lawrenceville, GA