IIRC the T-56 magnum uses two separate gears for the 5/6 driven unlike the older T-56 where they were a single unit. Theoretically, you can get the 2.66 Magnum and buy the .50 6th and add is after the fact. I had priced it out prior and I believe it would cost about $450 in gears. That way you have the short ratio 1-5 and tall ass 6th that is awesome on the freeway.
I run the 3.55 8.8 and it does make 4th pretty tall. 149MPH or so. First is still useless. Taking off from a stop is easy. My Ls1 had no issues with the 3.55. The 440 is obviously better. That being said, if I were staying F-Body T-56 I'd go to a 3.73 like Mark suggest and get much stickier tires. Finding an 8.8 with a 3.73 is almost as hard as finding the 3.73 MPV. If you go 3.73, don't bother with the Mazda diff. Go 8.8 and abuse the crap out of it.
