"However....the old banjo nut is longer than the new one and has a smaller hole
in it, whereas the new banjo nut on the top is so short that we can't get the
old banjo fitting around it and have threads to screw into the filter base. The
new one also has a bigger hole in it (which is why, we're assuming, we were
having high rev issues as in our previous post). Does anyone know what banjo
fitting I need for this?"
Can't help you with the fitting issue but I wouldn't think the flow volume through the filter would effect the high rev issue on start up. If it is not a cable or linkage problem I would suspect a high pressure pump / governor problem. As asked before, did the engine ever run properly and has there been any recent work done on it?