The engine isn’t the problem (other than weight) it’s all the supporting systems and PSRU. Start reading about torsional vibration and it will become very clear that they are extremely difficult to make reliable.
It does take specialized career skills, instrumentation and money. One can hire engine consulting companies like Ricardo, AVL, SwRI to do the work but it is in the arena of engine and installation development and tough to do on one's own, even with the knowledge. Not to mention the accompanying knowledge to design gears that last. These are not black arts, but not something one can quickly pickup, or get in forums and magazines.
I spent much of my career in the world of engine design, testing, development and applications. This is a daunting task to do well. I thoroughly enjoyed learning the processes and fixing all the issues (nothing is perfect). I had thousands of engineers within a phone call or 10 minute walk/drive to call on and learn from.
If you want something a cut above, consider the IO580 lycoming. It has the 390 bore/stroke and more than 300 hp. It would get excellent fuel burn (MPG) leaned at cruising at normal RV10 speeds (fast). It would fit, have warranty, be reliable, and special. Probably the same end cost as a LS3 conversion, and pretty cool.
If you do choose the LS3, then manage your expectations and employ the very best technical (experienced) people to help. Use proven components if at all possible.
Have fun with what ever avenue you take.