Single 2bbl. Webers beat SUs on two counts only---Much better starting when it's really cold out, due to having a real choke, as opposed to a "mixture enrichment device", and a smoother, more consistent idle. In terms of drivabilty and throttle response, a properly set up pair of SUs (either HSs or HIFs) are clearly superior. That said, there is plenty that can go wrong with the SUs---start by making sure there is no play in the throttle shafts, that the needles and jets are not worn, and that the jets move up and down freely in response to the "choke" control. Many SU problems can be traced to the linkage that connects them---play here causes uneven throttle opening, resulting in pairs of cylinders receiving different amounts of fuel---not the recipe for smooth running.

Now that I better understand your situation, my recommendation is to get a good B18 or B20, by whatever means, and a set of SUs that are not worn out. Ideally you want the early type aluminum intake manifold, and a separate cast iron double downpipe exhaust manifold. The one piece iron manifold with extra throttles in it is best avoided. I will second Lee's opinion that your current engine should be evaluated more thoroughly before being condemned. Redoing the head might well be sufficient. In any case, you will want to have hardened valve seats installed. 74 and 75 heads had them from the factory, earlier heads require them for use with unleaded gas. In fact, valve seat recession could be part of your problem. To check for this, remove the rocker arm shaft and lay a straightedge across the tops of the valve stems. They should all line up. If one or more of them (nearly always an exhaust) protrude, you have eroded valve seats. While it's apart, turn the engine over by hand and measure how much each pushrod rises. These should also all be the same. If one (or more) come up short, you have camshaft wear---the one weak point of the B18/B20 design.

The OHC alternative requires a lot more figuring and minor fabrication than rebuilding an OHV engine, especially where the fuel system is concerned. You would either have to install a complete fuel injection system or head up to Canada for a single Stromberg manifold and carb for the OHC. Then you would have to come up with a linkage and rework the exhaust downpipe, among other things.