273 or 318 or both help

you can get a running la or magnum 318 or 360 for less money than fixing either engine that you have.
Around these parts that might not be true, from his description. If the 318 is re-ringable, or maybe doesn't even need rings, I'd drop that 273 crank in and do what it takes to freshen it up a bit and go with it. This of course depends on the actual condition, whether it needs a valve job, and miles etc. Re-ring kits are not expensive.

A magnum is not a drop in. Because of his old converter he'd need to obtain/ make a converter snout bushing for the magnum, as he would if he used the 318 crank