If you're getting oil in the coolant or coolant in the oil you can be 95%-99% that you have a head gasket leak or a cracked block or head...
Did it overheat or run hot recently? Mine didn't when my head gasket went...they just went... my problem was different though... i wasn't getting oil/coolant contamination...i was getting air bubbles which were leaking into the coolant system and bubbling out through the radiator cap into the reservoir...either way, both of our problems seem head gasket related.(i know mine was)...
I did the job myself and i have to say... it wasnt as bad as i thought. the only thing i will emphasize is organization...find a place to put everything you take off and put every series of bolts in a bag and label where they went (this helped me since I had to remember where everything went a month and a week after taking it apart)... just get all the gaskets and seals to do the job and take your time...
I also did a valve job on the heads and hand lapped the valves... she runs much better since the valve job, but, unfortunately for me, I am still getting air bubbles in the coolant

I suspect a cracked head or block(I have been told that cracks can be very hard to spot, which might be the reason that i didnt see one)......all was not lost though...it was a tremendous learning experience for me...im hoping it will run for a while like this and i plan on driving the engine till it stops running again....then i will most likely get a rebuilt or used engine.
if you have any more questions just IM me on AIM or e-mail me at: HOT ROD 85
Good Luck... and i hope you have a better outcome than me...