Also, one thing to think about is the resale after all is said and done. Even if it's overpowered and you can't get traction, a 340/360 is always going to help things sell compared to a 2brrl 273.
I wanted to big block my car when I first bought it. But then after realizing how much is involved in having a streetable big block A-body I backed out. Torsion bars,disk brakes,motor mounts, is top of list of the things I would have to gather.
I would hate to remove the 273 because the VIN number shows that the car came with the 273. Now I'm not totally sure its the correct motor that came with the car however but it might be the original. I have thought about a nice built 360 would cheer things up a bit but I would not get rid of the 273 altogether. I believe the 273 are slowly getting hard to find but I could be wrong.