I was also looking at 255s for the rear as well. With the 3.73s, a 27" tire, and an A500 in O/D, the car should turn about 2400 rpm @ 75 mph.
Heres a question though. Will be running a clutch type limited slip. My spare tire that barely fits the well is 25.5" diameter. Lincoln town car spare on an aluminum rim. Will I run into a problem quickly wiping out the clutches if I have to use that spare on the back of the car since with it bolted on I will have 2 tires turning at different rotational speeds. That would be my only reasoning behind staying with a 225/60 at all 4 corners since that is a 25.5" diameter tire and will match the spare diameter wise. Nobody thinks about that ****. But real world situation, you dont want to be stuck with an unusable spare and a flat rear tire.
With 225s all around and my gearing I am looking at close to 2600 RPM @75 mph