Radiator Fluid(white smoke) via exhaust, Temp staying low

1981 Datsun/Nissan 280zx, 120k miles:
Initial problem: Water pump shaft broke causing fan blade to break and putting a gash in radiator
Results: Car overheated (300-350+? Degrees f)
Water pump, Fan blade, and radiator replaced.

Current problem: Temperature gauge on dash never gets past 100 degrees f. White smoke comes out of exhaust believed to be Anti-freeze. Pressure builds on radiator which has caused the overflow cap to bust off and fluid spraying out while driving. Pressure is always noted on radiator no matter how long engine is running. Thermostat has been replaced. Oil in car is not mixed with anything, oil pressure has remained constant. I am losing water through the exhaust. When I drive very short distances(1/2 mile or less) and idle or turn off the car smoke is noted coming out of the exhaust manifold, but after longer distances(1 mile+) only white smoke out of the exhaust. The car is losing approximately a half gallon of water per 30-35 miles driven. I have been told by a shade-tree mechanic that it could either be a cracked head, or a busted head gasket. Looking for more informative advice on the subject.
Help much appreciated.