things I can think of that would cause issues: weight of new engine ( front ifs torsion bars may need to be beefed up to handle weight. if there was a conversion kit to coil over that would be awesome ), motor mounts ( if they aren't the same then fabrication will be required, if they are the same they still might need to be beefed up), wiring harness and other similar interfaces are more of an obstacle than issue, if you decide to do it I would get a fully dressed engine but you also gotta consider stuff like radiators and will you need a new computer.
it would help to know what year and model vehicle you currently have. I know mopar has sold factory crate hemi's fully dressed in multiple sizes. if you remember the gen 3 srt dodge pickup with the v10 viper engine. a buddy of mine bought a wrecked srt 10 and swapped the engine for his 5.7 and it was a plug and play, everything fit perfectly. only issue was transmission because the srt is standard tranny.