273 LA bored .040 change in performance?

A .040 overbore by itself won't make a difference you can feel - just from the overbore anyway. Returning a worn cylinder to round, a fresh hone and fresh rings can add power through better cylinder sealing, and higher compression pistons can also add power, but the gain from just the overbore, probably 3-4 hp.

Formula for calculating the volume of a cylinder is pi times the square of the radius times height. Pi is rounded off to 3.14159 - or rounded ever further to 3.142. R is for radius. Radius is one half diameter, i.e., one half the bore. Because cubic inches measures piston swept area, H means stroke (not total height of the cylinder). So: pi times the square of one half the bore times stroke. Then multiply that number by however many cylinders you have.

Or - an easier way if you're mathematically challenged is just use this calculator: Calculate Cubic Inch Displacement

At that size motor, a .040 overbore will gain six cubes, as said above. A larger motor will pick up a few more than six cubes from a .040 overbore (a 440 gains eight cubes), smaller motor less.

As also pointed out above, every 1965 273 came stock with a solid lifter cam, just like all 318 poly motors and all slant sixes (until much later). If your 273 had a hydraulic cam when you pulled it apart, it was not original. Also, dunno if your 273 is hi-po or not but when it was bored oversize, it most likely got low compression pistons. That would be a horsepower loss if your 273 originally had 10.5 to one pistons, and the small overbore sure wouldn't make up the difference.