First, even though you have a 428, your 390 cfm carbs may be quite enough for the street. 390 x 2 =780 cfm which is more than your 428 can possibly use even if 100% efficiency is considered unless you intend to visit 7000 RPM repeatedly. You might get better results to have your present carbs properly jetted and set up by a reputable shop rather than purchase bigger carbs. Even if you decide on bigger carbs, remember that your main carb is still the front one and if I recall correctly the idle circuits and maybe even the jetting may be a little different for the main and secondary carbs. You want to remove the choke assembly from the secondary carb and you MUST install the vacuum pot covers intended for dual quad setups which have a vacuum equalization hose connecting the two secondary pots.
http://carlsfordparts.com/parts_imag...VAC%20TOPS.JPG
Lastly, if you are going to use a progressive linkage like the original dual quad medium riser linkage, pay attention to the holes in the throttle linkage plate that you install the trunnions in. The pivot radius on the secondary carb is shorter (closer to the throttle shaft) than that on the primary carb because the secondary carb needs to open quicker than the primary carb. On the original dual carb setups the mounting hole for the trunnion on the secondary carb is closer to the actual pivot point than the trunnion on the primary carb. Take a good look at the trunnion positioning in this photo.
http://carlsfordparts.com/parts_imag..._induction.JPG If you use Holley 1850s or any other pair that has not already been set up for dual quad induction you will need to pay special attention to placing the trunnions in the proper position or the secondary carb will not open up 100%.
Anyway, before spending $1000+ on new carbs, find yourself an induction expert and do your homework.