Thing to do with the lightweight is remove the tap from the engine altogether, these are prone to seizing up and restricting flow of hot water even when working correctly. You won't get to warm in summer of course because the top will be off.
With both heaters make sure the core's are clear, they tend to collect rust and muck restricting the flow of hot water. I flushed mine through both ways with a hose pipe several times. The muck that came out was unbelievable. Keep going until the water runs clear.
The heater on a 110 should work very well providing the core is clear. Check the foam seal of the heater box to the bulkhead, this can disintegrate and let could air from under the bonnet through.
If it's only you sat in the 110 most of the time, have you tried blanking off the rear section. Friend of mine hung a piece of clear polythene from the roof just behind the front seats, effectively reducing the cab size and increasing the heat in the front. If you've got a soft top, a proper fume curtain is the way to go.