A 318 will go right in I have a 65 cuda with a 66 318 in it 904a trans push button in the dash. Seriously, check the valve train out first, if it wuz me I'd keep the comando in it if at all possible, if you swap out try to get a 68 or older block, the newer ones are LA blocks and are stroked a bit different the head deck angles are different as well. I had a 69 la block in mine first and sold it, just doesn't perform as well. 273 parts are still fairly easy to get you might have to snoop around a bit. AND KEEP THAT 273, there are folks that would love to have it (me included :^O)