Are you sure you don't have a vacuum leak either between the manifold and head (ie maybe there is enough of a difference in the flange thickness of your intake and fentons that the washers aren't pulling square) , or at one (or both) of the carbs?
Are those the carbs with the power valve that needs a flange gasket with a cutout (and if so is the cutout oriented properly)?
Definetly check the valves (compression check too if you can).
You say the cap & rotor look good, are you still running points? Not the most likely culprit but a set of points and a condenser could do that, or an excessively worn distributor shaft or drive might as well. (again not the most likely, but wierder stuff has happened)
If all is good there and you want to try new hardware as opposed to a new overhaul shop, then there are 2 options I can think of, Langdon's Carter/Webbers and adapters, on your current manifold, if you want to stay with carbs.
If however you are interested in EFI, then might I suggest a Megasquirt kit and 2 single barrel TBI units off early to mid '80s econ-o-boxes, each on a home made adapter and bolted to your current manifold.
There is a thread here about retrofitting TBI onto a factory single manifold (I'll see if I can find it for you). The dual would be slightly different in the plumbing and wiring, but not exceptionally so.
Either way should be cheaper than the Clifford setups you mention, if you do most of the work yourself.
If it's what you are after, the Clifford setup should make more peak power though.
