The move to the 670 is the right move, but I think I would change that rear gear to 355's. Your WAY too low in rpm's to be hitting the traps at 4200 RPM. I would even consider trying a 750 cfm holley for a couple of passes with the cam you have before I went changing it out. You should be hitting the traps at about 6200 to be getting all of the power band, and your strangling those heads with the small carb.
No flow- no go....just a thought................Jim