There are a number of threads re: fuel gauges. The one on my 90 is far more accurate than on my 4/4. I go by mileage on the Morgan (reset trip every fill).
As for the rad, if you've bought a car with the plastic inlet/outlet pipes then going for a Mulberry rad makes sense. If your car is post '06 (4/4 or +4) Williams also do an ally rad...