rebuild it or swap it?

I recently bought a dart 270 vert with a 273. When I purchased it I was told the motor had been rebuilt before but after getting it home and removing the valve covers that is obviously not the case. Luckily I have a spare 318 ready to role. Should I try to rebuild the 273 or just swap it out? I am not too concerned with originality. Will it swap right out or will there be mods involved?