273 or 225?

Depends on what you want,
Tough decision. Cost would be #1. It all has to do with budget. Ask yourself, would I be happy with a Slant ?
Am I really a V8 guy ? Last is what plan for the future of this car ?
You can't lose either way you go.
This about sums it up. If you have not invested anything in the Slant Six engine or the parts car yet, may be the time to get out the pencil and paper and list some hard figures.
Last thing is have fun with what ever you choose.