Fixing vs. Buying a New Engine / 273

The risk is that you may find more work in the 273 than you plan on so you'd be better off swapping engines.
You might be able to Find a good running parts car with a 318 or better a 360.
The 340 sounds like a good way to go too.
Might as well plan on the axle swap at the same time.
No reason to keep the 273 and there's some value in parting it out to pay for your upgrade.