Pretty good engine, but getting pretty pricey to rebuild. There is some info on the Jimmy v-6 here.
http://www.6066gmctrucks.org/
Some of the vendors onthe links page have some parts. http://www.stovebolt.com/links/parts.htm I think Kantor's lists some some engine parts, and probably some of the other vendors as well. With bell house, motor mounts and motor mount brackets from a v-8 truck, a small block chevy will bolt in.

Joe McNeel

http://mcneelnet.com/~jmcneel/59.html