Double OD is nice especially if you want to run steep gears and the T56 has more torque capacity so its not a bad swap. You can also go with a TKO 500/600.

 

Tesksid block should be fine , even the 03/04 iron termi blocks. But I know those are hard to come by. With a few upgrades on that termi, still with stock Eaton sc can make that much easy. If you can get every thing running in harmony. But some would just opt out and just go supercharger upgrade, as in Kenny Bell or Whipple, maybe even a centrifugal. I seen some interesting numbers on a dyno test comparing superchargers, Roots type, twin screw, then centrifugal test was all done on low boost between 10 to 12 lbs psi . Then finally with twin turbos. Put some forged internals in that 96- 99 cobra and slap a centrifugal supercharger on it should have no problem for 600 horsepower. Good luck