The 273 I had tested would have gone .060 but it looked like a nice block with little core shift. Without sonic testing I would not go over .030. As for useing those heads you are gonna have to notch the bores to get valve clearance. At .060 over a 1.88 valve barely clears, I do not think it would have cleared a stock bore, that is how close it was and a notch would still be good for airflow. Those aluminum magnum heads with a 1.92 valve are gonna need a notch no matter what you bore the 273 out to. Now if you went with a stock iron mag, the valve job is terible in stock form for a 1.92 valve however you that same bad valve job will allow you to machine the valve head down to a 1.88 diameter and with a .060 over bore you should get enough clearance to run but again a notch is still gonna help air flow tremendously. The only other option I can think of to use the aluminum magnum is to have a smaller seat installed and machine the valves down to that diameter.

As for strokeing it I know a 4 inch will fit with SCAT I beam rods but pistons are gonna be custom only. I mocked one up with a stock piston and just had to clearance the skirt to clear and it would rotate, check very closely at the rod to wall for clearance, not as much clearance there as a 318 even. With notches at the bottom of the bore you may get a little more stroke out of it and it can get around 340 cubes the really hard way and with very shrouded valves so it will not make the power a real 340 will.