First thing, what has been done and describe motor
Carb, igntion, plugs, plug wires changed, modified etc.
White smoke out of the exhaust indicates a blown head gasket, or a cracked block. Coolant is leaking into a combustin chamber.
Don't go for the worst just yet. Carefully diagnose all components. Points, condenser, firing order, fuel (make sure it is fresh, water in the gas will also signal a blown head gasket)
Take the easiest, and cheapest look ats first.
Keep us posted
