Will a 1985 Chevy V6 engine and transmission fit into a 1983 Chevy V8 truck?

Yes, if you are refuring to the 4.3L V6, Any Small Block GM Transmission will work with this Engine-Transmission combination. Also I do believe the Buick 3.8L V6 will work as well. Any other V6 will not work as a direct bolt in. But anything can be done as long as skill and finance allow.