The 250 with the 6" rod swap would be a better combo to build. You will be introducing a much more severe C/L ratio into the engine by putting a 292 crank into a 250 block, and you will be making the problem much worse.
In a perfect world, you would need a piston with a compression height between 1.380"-1.385" to use a 6" rod in a 250 block with a 250 crank to allow a light cut off of the deck to bring you to near zero deck height. That's also assuming your using a block that hasn't ever been decked before.
