Scott,
According to this page
Pg 148 of 1929-57 Master Parts Catalog the pieces you seek are as follows:
Underpan: GM# 3725088 (they often come up for sale on eBay, bit couldn't find any active listings tonight)
Extension: GM# 839180 (there is one currently listed on eBay - search using this part number to find it)
Bolt w/washer (4 each): GM#839301 (Online references show these as under new GM# 3817006 - 1/4"X20, 7/16" long - readily available from various sources)
Just a note that you can always post an ad in
Truck Parts Wanted as there are often fellow Bolters that may have what your looking for too.
Good luck in your search!
