273 LA bored .040 change in performance?

Since it's not original, and you're gonna re-do the motor anyway, swap in a common-as-dirt 318 block. The extra 45 extra cubes will gain more performance than getting (expensive) higher compression pistons for your 273. And everything else you might want to use with your 273 will work with that 318 block.

PS: I know from another thread that 273's have their defenders, theirs run great, blah, blah, blah. And yeah, you can build a decent running 273 if you really want to. But there's a reason people aren't even building 340's and 360's for the street anymore, given how inexpensive stroker cranks are these days. All other things being equal, the large the motor, the more power it will make.

Only reason I might recommend a mild 318 over a something like a 416 stroker to replace a basically stock 273 is that you begin making so much power you overpower everything else you have - trans, rear axle, heads, intake, exhaust, etc - so you end up having to upgrade everything else to make the extra cubes work.