Engine is pulled - 273 rebuild or replacement 360 crate?

[Mean416]

360 over the 273.

You could be an oddball and use the 273 heads with bigger valves on the intake side and some Port work. It'll be a stump puller off the line and still work well to about 5500 rpm.
In other words it'll make the horsepower of a 340 cammed 360 but have a hell of a lot more torque, probably to the tune of about 60 more foot pounds.
I used those type of heads and did exactly what I mentioned above on a 340 and it made 305 horsepower and 380 ft lb of torque. Near stock 340 cam but as a solid. It was .473 lift 110lsa 227 @.050, minus lash was about .458 or .460 lift... or an honest .450 at least after rocker n geometry losses.

I don't believe a 360 with a 273 top end would make more low end power than a 360 with a 360 top end.

 

[Mean416]

Get a 5.9 magnum junkyard motor. Call up Oregon cams and get a regrind. Get a cheap air gap and a decent carb. You'll have a 350 hp engine with a broad power curve.