You claim it's not going to be "just a drag car" which indicates to me that it might see a bit of track time though. If you have access to an inexpensive 318 block, I'd use it and keep the original 273 block in cosmolene stored away for any potential future restoration. I'd then reuse all the other 273 parts in building the 318 so you can keep the original look. As Professor Fate mentioned, you'll need to do something about the mismatch of a A500 converter to your small register 273 forged crank. Either machine out the end of the crank, or, if possible, I'd get a custom converter built instead. Another option would be to use the 318 crank, which they made in both cast and forged. Either one will handle the horsepower you'll likely end up with.