new engine

If it was me I'd say stick with a 318, find something local you can hear run. If it's god, that will accept all the psrts off the 273 and literally bolt in without raising the spector of exhaust flow and fitment, mounts, oil pans, etc. It will look factory 273 but be a little better. If you're not a gearhead, and will be having to rely on a local place to do it all, simplest is best IMO.