Would you install a 2.15" intake valve?
If the bowl can be opened to the usual 85%, yes. Improves low lift-flow. Frequently taking off more metal also allows more radius in the turn (depends on the casting).
I agree - the math (adding .3 to the rod compensates for .3 less CD), and a 250 could probably stand being slowed up around TDC a bit.
I have a feeling that the valve reliefs on the LS don't match the 250 in size, placement or angle (you may luck out, and they're too big so no work needed, although minor compression drop).
I don't see any reason why a cup couldn't be added to an existing solid dome as long as the critical point (typically determined by the span of the dome over the pin bosses, position of valve cuts, blown or N/A, etc.) isn't below .210" or so.
The floor of the cup should be "flat" (90° to the stem axis) under the valves, but not a big deal. The sides can be done with a die grinder if you're careful.
Set up the piston in a mill vice, and the math is easy. Just mark out the area to be cut, typically 3/16" minimum away from the edge to prevent overheating, and matching the quench ledge, Now transfer to graph paper and guess the area. Area × depth = volume.
