Dave, I'm thinking the Magnum heads may be a little big for a 273, since it is about 85% the size of a 318 (the smaller of the two Magnum headed engines). I'm thinking 302's with a good valve job and a little touchup blending. But, you would be giving up the 1.6 rockers. A custom cam could make up for that.
With the O.D. trans the 2.76's may be a little too tall, but maybe not.
Just thinking out loud