"drop in" small block

The 340/360 gets more complicated ( oil pan, motor mounts ). Later model 318 will be the closest to "drop in" all around. Differences at timing mark versus water pump outlet. Balancer versus pulley bolt holes. The more you can move from the 273 to the 318 the better.
Put a bushing in that larger crank register for your small nose convertor.
I just couldn't imagine you would toss out the original 273. I've got one under foot here too. Pull bar and socket lays right there with it to turn it at least every 2 weeks so it doesn't seize up.