tires for 15" Rallye wheels

I know I'm doing this a little backwards but I didn't have much choice. I was driving my '65 Barracuda home when my brakes failed. Now it's parked and I'm installing front disc brakes a la Scarebird. I need/want Rallye wheels and Wheel Vintiques sells Rallye wheels in the 15x6 size.

I've measured my stock steel wheel/tire diameter to be 26.1". I want to keep the same diameter combo for the 15" wheels. The question is: what size of tire can I mount on these rims without interfering with either my speedo or risking rubbing the tires on any internals?

Does anyone have 15" Rallyes on their early A-body? Any glamor shots?

-marcus