chevy sprint - blown head gasket

You will need to do a compression check to verify a blown head gasket. The symptoms you state does point to a blown gasket, but it could be the aluminum head too. In any case, you will have to take the head off and inspect. You can cleam the oil and coolant passages with compressed air.

The goo probably made it to the oil screen and set off the oil light. Are you still driving with the light on? I'd flush only the engine with hot water and diswashing detergent to loosen the oil. The gunk will end up in the grill if you separated it from the motor first.

radiator is fully of sludgy grey brown goo - and the water was way low and the car overheats seriously on just a fairly short drive and now the oil pressure light is on, too ... there are no external leaks.

completely in order to remove the sludge from the cooling galleries - or is there some way of cleaning it out adequately without disassembly? I'm sure the radiator needs complete cleaning.