You are right about the vacuum secondary/mechanical secondary carb operation. If that 355 is not spinning past 6000 rpm, it would seem that the 650 VS carb he has, should be just about right. You say he has fuel stavation problems. Has he checked the basics, fuel line size, fuel filter, ect.? Has he tried tunng the carb? Maybe just a simple carb rebuild would help?
I'm running an AED 750 mechanical sec. carb on my 68 Camaro, but it's a manual trans car. You might want to try giving them a call for a recommendation, not cheap, but they really know their stuff.
http://www.aedperformance.com/Carburetors.htm
Mike