Carb choice

First off, you are already down on torque, why in gods name would you stick a longer rod in it? Thats for guys with head and intake that will support 7 grand and up. As it is, your rods are too long from the factory to make any bottom end grunt.
If anything, turn the pistons around in their bores to pick up a tiny bit of angularity.
If you are dealing with a major bog when the secondaries open, you're not likely giving that thing enough initial timing.
My advice, if you want to run a big carb? Lock out the distributor and set it for 32-34 degrees. Big oomph when you hit the pedal.
One other thing. Once you get rid of the bog, you're going to find that 500 is jetted way too fat for a 225 inch engine.
Ive got a 600 Eddy on mine, and found that .098s in the primaries, and .081s in the secondaries are just about dead on.
Fine tune it with drop rods