I finished the carb switch, so here's what I ended up doing. I bought a pair of Holley 4160 600 CFM carbs, one with an electric choke and one without. These are aluminum single fuel inlets with vacuum secondaries so they mimic the originals in function and are slightly lower CFM. I took the advice for the 1/4" phenolic spacer and related hardware to minimize the vapor lock issue. The new carbs come with a fuel fitting that sits sideways along side the fuel bowl, so that needed to be replaced with a straight out the side fitting in order to use my current fuel lines. The rear, manual choke, carb has a large vacuum port that points forward between the two carbs. That port is in the way of the throttle linkage, so it was removed, tapped, and plugged with 1/8 NPT plug. The base of the carb is also shaped forward where that vacuum port is and that forward shape is in the way of the flat mounting bracket for the throttle linkage. So the bracket had to be modified for clearance. I cut the bracket to mount on each side of the carb with no center section. It is still solid, but the alignment is critical to prevent binding. The next fiddly piece is the new carbs float adjustment screws are round and tall, so they interfere with the air cleaner base. I tried a 1/2" spacer, but it was too tall for the hood to close--touched. I made a spacer about 1/4" thick from some neoprene rubber and that allowed good clearance. Start up was uneventful, ran smoother than I can remember. The idle was much too high and it took a little fiddling between the idle adjustments on the carbs and the linkage to get that right.
I am now ready for a road test when the weather turns dry and will know how the vacuum secondaries work under load and if they need any adjustment. Overall, I am pleased with what I have right now. Wonderful not to have any leaky carbs! I figure I will send the original carbs out for a rebuild while that can still be accomplished and store them for someone who wants to go to a fully original setup. Thanks again to all of you who gave me part numbers and suggestions on this project, they were invaluable. Couple of photos attached.
Charlie D