make sure you maintain your cooling system and add the supplemental tabs to the lower radiator hose! They are Extremely important in the 4.9L.
Amen to that - I replaced my thermostat, waterpump and radiator cap about three weeks ago - flushed the cooling system at the same time and replaced the coolant - neglected the barsleak and had all kinds of problems (mystical loss of coolant, the occasional heat spike due to air in the system) - put the bars leak in a few days back and now she runs cool as a cucumber (which I assume is pretty cool) and the coolant level isn't dropping.
I presume that this is due to leaking at the juncture of the aluminum block and iron heads which, having different densities and therefore different thermal expansion rates, must weep a bit from the gasket surfaces.
Also - don't be disappointed about having "just" a Seville - it's a heck of a car - fantastic on road trips - I doubt that I could go back to my soft top jeep again (although when the snow covers the roads my opinion may change!)