Are Oldsmobile 350 455 heads interchangeable and if not please explain?

Yes, but with some note worthy issues. The heads will bolt to the block, but you will have a large intake port mismatch (the 455 intake ports are about .300 taller than the 350 ports.). You can not swap intakes to cure this, since the 455 has a taller deck height than the 350. You can however, port the top of the intake runners on a 350 intake manifold to get them closer to the 455 size (an Edelbrock Performer RPM can apparently be ported to match), or port a 350 head to get it closer to the 455 intake. Be careful to not remove too much material. There is also the problem of compression 455 heads are listed as 72cc combustion chambers, but usually measure closer to 80cc. 350 heads are listed at 60cc and are about 60-65cc. Swapping the 455 heads onto the 350 will loose about 1.5-2.0 points of compression. Results will be around 7-7.5to1. You could do it with coustom pistons to get back the lost compression. 350 heads on a 455 will raise compression too high for pump gas, and have smaller ports and (usually) smaller valves, not good for performance. Lots of Oldsmobile info at this site: http://www.442spot.com/cpierce/oldsfaq/ofhed.htm