Long time lurker here.
Tio540 and the tsio 550, both are options for my new exp project. I'm at least a year out from needing a power plant, for now a big hunk of cement is all I need for what I'm working on.
I've read about the cam gear issues on the continental as well as the shock issues on both. I've also read about the cylinders never making tbo on the continental, with the Lycoming cylinders in second place when mistreated.
How about the other possibilities, like mods to either or both to increase dependability? As this is experimental I can do what I choose.

What are the other options aside from these two? I've read a little about the 580 but it seems like it's a river to far in terms of availability.

The (5 year)project goals are 1. Dependability/ safety 2. Minimum ceiling lower fl's and 3. HP 320
I truly like the availability of the tsio 550 but I've also read a bunch about it at annual and it's in flight failures, (seems to be brs on cirrus's was good thinking).
So, do you guys have any real world experience with either or both you'd share?

I have a TIO-540 J2BO firewall forward Core for sale. 1800 smoh. It is installed on a 206. Was test run to idle a month ago.

I dont have experience with the Lycs. Flew against them running back and forth LA to the Grand Canyon. Same loads and speeds in the 402 with TSIO-520 and would do the same job with less fuel. I installed the fresh TSIO-520 OH engines myself, broke them in, ran them nicely, kept good airflow over the cylinders, Cruised 55-57% power and leaned. I put 1000 hours wheels up on that plane, was told it made TBO without needing a cylinder.

I have experience with the IO-550. It makes more power than the 520 but also more heat has to be rejected with the same cooling system. You either have to remove it with more air or fuel. It looks like they tried to move the heat with the oil and I was chasing oil temps in my setup. Cessna 206 IO-550F for Skydiving.

I think this depends drastically on the mission. At 320HP, you're looking at the high end of performance from either engine for reliability. If its in a heavy hauler, I think that a angle valve downdraft 540 will probably be easier to do reliably than a TSIO-550. The 550 needs a lot of airflow to keep it cool and you're locked into a given oil cooler size, since it's part of the engine. That said, unless there is already an engineered FWF solution for your airplane, you need to have a very solid understanding of how air flows around a cowling to make either engine work reliably. Intercoolers add another complexity that you may or may not be dealing with.