My choice would be a 4.25 stroke, 6.535 rod, piston set up for Max compression you are willing to run. Cubes and compression are very effective in upping the hp potential of your build . .043 top and second rings, gas ports, don't worry about rod length or piston pins height being her n the rings. Check to see if it is possible to cut the closed side of the piston only. That will create a bit more compression and cost about as much as a thick cometic gasket, or less. Watch out for valve to piston clearance, or the need to add some.
If you are going to spend money to build a good motor, why not build it to make an extra 75 to 100 hp by employing all the changes you can that add power? A b body isn't light. Build the 512. Use narrow rings. They work, and live. What heads do you plan to use?