I thought the 3400 was just a bored out version of the 3100 with better heads? The lower intake shouldn't be an issue with the upgraded gaskets though. For the 3800, from the research I've done, you need everything from the subframe up. The top engine mounts fit right in, the axles from the 3800 fit the Montana, you'll have to keep the power steering rack from the Montana, if it's the older model I think all you need to do is splice the wiring harness at the fuse block from a gp, if it's the newer then you'll need to splice at the pcm and use a park avenue pcm, custom power steering lines, splice fuel lines, coolant hose should work from gp, heater core lines from gp should work, custom ac lines. If it has the 4T65E stock, keep it, while it's possible to modify swap for 4T60E,just don't. That's it. Stock tune works fine. You can get all wiring schematics from auto zone website, but let me know if you need help!
