1993 Trans Sport 3.8L Oil in Anti Freeze

The van and motor have approximately 400,000 km on them. I went out yesterday and removed the rad cap, and low and behold there is a layer of chocolate milk on the top of the AF. I am quite certain we have oil in the rad, so that could mean intake or head gaskets leaking.
I have no idea how long that problem has been developing. I had a leaky heater core for quite some time; I used some stop leak then. When I replaced the heater core I don't remember the oil in the AF.
I wonder if it's worth trying stop leak now to seal up the oil transfer? Would I use engine oil sealer, or coolant system sealer?
Has anybody changed head or intake gaskets on a 3.8L? How tough of a job is it being tucked away in the engine bay like that?

Can you pull the lifters without removing the heads on a 3.8L GM engine?
There is a tool, a small slide hammer that will reach through the push rod holes and pull the lifters out of the block. I wonder if it will work on a GM 3.8L engine?

Are you sure it's not automatic transmission fluid?
Do you have the milky brown substance in the oil fill cap?