LA 340 Intake Manifold

For the amount of work you need to do to the 340 to make it happy on the street, I would just find a good 5.9 Magnum, have the cam reground (Oregon Cam Regrinders), throw a Chinese AG clone manifold on it, and rock it.
By the time you rip down the 340, change out those 12:1s to something more street-friendly- and possibly have to re-balance (do you really want to hunt down AV/race gas every time you want to run to the ice cream parlor?), change the cam to something that generates enough vacuum to operate those power brakes, change out the manifold to something with a more street-friendly RPM range, you're basically doing an engine rebuild anyway.
The added stroke of the 5.9 will help build torque for that B body, and the stock pistons are pump-gas friendly. Normally, good 5.9s require nothing more than a touch-up hone to the bores when refreshing the bottom end.