Sorry for the double, figure out what your shaft's critical speed is, then, assuming your T56 has 1:1 4th gear, look at your rpm vs mph. That tells you what your driveshaft is spinning. My 1/2 critical speed was around 70mph, have my cutoff set to 140mph, driveshaft would spin 7000 rpm! Luckily I'd only gotten to 132, it was sketchy enough for me.
You can use this calculator to look at rpm/mph/shiftpoints, I love it, helped me visualize my downshifts/upshifts:
https://www.blocklayer.com/rpm-gear.aspx