Usable power for the street?

Hey VR, post some pics of your Barracuda. Everyone on FABO loves pics.
You mentioned your existing 273 and transmission, but didn't mention what rear end is currently in the car. That's going to be another prime consideration. A 7.25 differential may not hold up to your 273 Commando. A 8.25 may be good for a stock-built 273 but may be marginal for a stout 273. If you want to go to a built 360 or even a stock 5.9 Magnum then I'd be looking at a 8.75 to live behind either of those.