Have tried all of the above and have a 4-row 'Excel' radiator core and a small shroud around the (Texas Cooler) fan. Temps stay around 180deg when cruising but always creep up at idle on a hot day. The one thing that worked on the idle temp issue was a Hayden 7-blade stainless steel flex fan. That thing really moved the air, especially at idle, but I got tired of the howling noise it made and put the Texas Cooler in. I have a sleeve on a 180deg Robertshaw thermostat and, frankly, don't think it buys you much (maybe a degree or two at most).
With a std. 7-lb cap you won't boil until either 226F or 233F (forget if you get 2 or 3 deg per lb). The rise in temp when you shut down is called 'latent heat,' which is kind of a misnomer because when you shut down the engine is still producing the same amount of heat but it's not being dissipated by the cooling system. Perfectly normal--that's why on cars with thermostatically controlled electric fans the fans run for a bit after shutdown--but 230deg seems a bit high (then again, I haven't driven my Healey in FL so that might be normal).
I'm convinced that not only were Big Healeys under-radiated but they also trap too much engine heat in the engine bay. I put header wrap on my downpipes and there's more heat coming out the exhaust that presumably would otherwise be heating up the engine bay, but can't say for sure if that's helped coolant temp. I think ceramic coatings--e.g. JetHot--applied to both the manifolds and downpipes should help a bit (your feet might get even hotter, though :wink:. Also, in FL you shouldn't have to worry too much about freezing and could run pure (distilled) water--with Water Wetter or soluble oil to lubricate the pump--in the summer. Water does a better job of heat transfer than antifreeze. I suspect that would buy you a few degrees.