new engine

With a 360? .... Almost, here's why....

The 360 is externally balance engine, so that requires you to purchase a special B&M flex plate to mate with you neutral balance converter. There not to expensive at all.

Next are motor mounts. The simple way to fix the issue is to purchase "1" large nut to "slip over" the bolt of the motor mount. This takes up the extra space that the 360 mount is wider. It doesn't have to be exact.

A minor issue is the exhaust manifolds. The 273's will be a little choking on the bigger engine. If the 360 has exhaust manifolds, see if it will work on your car. Different year engines have different exit locations. In a worst case issue, reuse the 273 exhaust manifolds.

After that, maybe, maybe, the radiator will need to be replaced with a larger one. These can be purchased new. The 360 produces more heat and therefore needs more cooling capacity/ability. The old radiator may possibly be smaller but certainly older and perhaps a little gummed up/clogged from the many years of use.

The order of importance is in order. The first one is a must.

I would try and get a 360 engine. There plentiful, produce excellent torque and good HP in stock form. Simple bolt on's net very good power gains without going into the engine for a cam swap.

I myself was able to get 19/20 MPG's out of a 360 in a '79 Cordoba.
I only did bolt on stuff. An "H" piped 2-1/4 exhaust off the exhaust manifolds, 625 Carter on top of a OE intake, a K&N filter in the factory air cleaner, a vacuum advance distributor from MoPar and a MSD 6A box.
Gears were 2.76 and tires were 235/60/15's on all 4 corners.

Tune it well and cruise it easy. Enjoy!