The engine is almost certainly well used, it was second or third hand when it was fitted to the bus in the 70s, but it did no work at all until late 2019 when a small group of us decided enough was enough, after various people had fiddled with it to no avail, we had sump, manifolds, carb off and started on those again. It had air, exhaust, and petrol leaks galore. I am told it was honed and had new rings, valves etc. back in the 70s, as they were still then commonly available. I have seen pictures of this work being done. I was quite pleased that after a couple of weekends worth we got it to behave itself. You are quite correct, top has to be treated like an overdrive, it slots in at 30mph on the dead level. I am not convinced of the efficiency of the vacuum advance, but I daren't advance it up any further. Accelerator pump was just a thought as this is something it lacks. Being a fairly heavy 4 cyl engine, the revs are slow to drop, but the clutch stop is very effective, so this helps. Not much was done in 2020, so still early days really.