The v6 is going to be harder to drop in than a V8 as you will need to make your own motor mounts and if you want headers you will make them also. I doubt the magnum manifolds will fit an early A body, they barely fit the later A bods.

A 273 is a bolt in, don't make too much power and can easily be swapped for a 318 340 360 or even a 408 later when he is ready to lose the training wheels.

All obstacles aside I would do it if I was planning it to be the permanent engine because I like things different. I would not go through the hastle if the long term plan is something bigger eventually.