I believe the bore centers on the Chevrolet 3.4L V6 match the Chevrolet 250 L6. The 3.4L V6 appears to be the best cylinder head to work with for reasonable fabrication. The cylinder head bolt pattern is not exactly the same.
http://www.highperformancepontiac.com/features/hppp_0606_pontiac_sprint_six_engine_build/index.html
