someone help me with my slant 6 problem

Did you remove the thermostat? If you did, put it back if it works.
Have you pressure checked your radiator cap? Should be 16 PSI.
Was the cylinder head & block hot tanked/boiled out? If not, drain coolant and knock out a large freeze plug. If there is a bunch of junk in the bottom of the block, you can imagine what's in the top.
Is the water pump under-driven?
Back in the way back, when Dupont first introduced Xerex anti-freeze, one of its features was to seal leaks. It also sealed heater cores and valves. Never made a study of the /6 cooling system, but if there are small passages that could have been similarly sealed. (Thinking mass quantites of Barr's Leak.)
How's the head gasket. If it's bad, it could be blowing hot gas on the intake. If compression is good and even, rule this one out. Can also paint small areas of head to block seal with soapy water without bubbles. (Won't stay in place long enough to use on large runs.) Turn engine over by hand. If you got bubbles, you got problems.
Can increase cooling capability with three or four row radiator. Think most /6 had a two row unit.