Hello, I have a 1990 chevy k2500 with the 350 tbi. I just had the upper half of the engine rebuilt and it cost alot of $$$. I now feel like the rear main seal is leaking and its gonna cost about $700. What should i do???

try some AT 205 reseal, amazon sells it, and put it in the engine oil, often stops leaks there

is there anyway i can tell if its the rear main or the oil pan gasket???