I do not understand why you would want to run a 350 Carb? I believe it will cause a restriction for what power level you are wanting to get out of your 250.
If you cannot get the 500 carb tuned correct, I do not see having/switching to another carb is going to do some magic trick & all the sudden work correctly.
I do not think you have done all you can do to your carb, as far as tweaking it to run correctly. It's OK to say, I give up & it's beyond your limits, just find someone who can tweak it for you.
