O. K. First of all take the exhaust manifold off and start the tractor. Wear ear plugs. Does the white smoke stay with one or two cylinders or do they all puff white smoke. If it stays with one or two cylinders, take the injectors out of those cylinders and put them in cylinders that do not smoke. If the smoke stays with the injectors, get them checked, if not you have a compression problem. Get the head back in spec. Put a full set of seats in unless it is a Super head with the sleeve and put the intake valve recession at the shop manual setting and then set the stem height of the exhaust valve to match the stem height of the intake valve. You will probably gain 50 psi. Also  if possiable I would make a cylinder leak down test. Not that hard to do if you take the rocker arms off. 
One comment on the cracked head. If you take the head off and you DO NOT find any cracks I would start to worry. At this point in life (50 year old tractor more or less) they are all cracked. Now you need to know which cracks hurt you and which ones do not. If you take the head off I would also be interested in what your piston protrusion is. Confused, email me. get that all done and I will explain how to time a PSB pump.