You're thinking of the TR6060 which comes in the new corvettes and camaros. The "magnum t56" is a TR6060 with a standard slip yoke style tailshaft housing.
This is true on the speedo. However, you'd have to play with the gears to get it to track correctly with the z gauge I'd guess.
If you aren't making big power, an F body transmission will do fine for you. The problem lies with finding one that is in good running order for less than a mint.
On the subject of gears - go with the 2.66 - 1st gear set. this is a great setup for a street Z.
The .50 overdrive that comes with the stock t56 is ok, but depending on your setup can produce a lot of droning.
Solution: You buy a Magnum T56 and have it shipped to my house, I'll ship you my built T56! bingo! hahaha
You might be able to find a running F body T56 for around 1500, but its always a mystery unless you can drive it before. The magnum is 2500ish + bellhousing.
IF I could do it over, I'd absolutely have a magnum in my car - no questions asked. They're overall a better, stronger transmission, and they aren't "Mystery Mile"!
That's my $ .02