I received this info below about the oil pans, from one of the very knowledgeable Guys; Lewis Matkin, from the 6066 GMC website. Parts CAN be found on Craigslist; not always in your region or by direct order, but I have purchased many parts from the CL including my 401M.
http://www.adhuntr.com/ used to be Allofcraigs
http://www.mecanicapedrosa.com/
all
GMC V6 engines carry the same pan configuration. The only
difference is the depth of the pan. The 305 pickup engine has a shallow oil
pan and may not allow the oil-driven govenor spinner asm to be installed on
the pump. It is a 5-qt. pan. However, if he finds one from a
305-351-401-379-432-478 out of a 4000 series or larger truck it should work.
Most of these pans are at least 8-qt. capacity and will bolt up to any of
these engines
Best of luck with your search, I'll keep my eye out as well.