many factors:
what are you gonna put it in?
what are your expectations for the vehicle?
what is the axle bearing diameter?
leaf or coil springs?
or will you do a 4 link/ladder and coil-over set up?
what are the flange-to-flange measurements?
will you have to narrow (or lengthen ) the housing?
if you have no plans immediately for it, and you can get it REALLY REALLY cheap, buy it and sell it off at a swap meet.
I believe a 68 Tbird housing should be a "large" bearing, 4link, coil spring housing