I think your only local option would be kit from TAT...your best option if going with new IO550s would be just order direct from TAT. They do a Gamispec TNIO550 that has Millenium cyclinders, ported, polished, balanced, pastuerised etc etc...the milleniums have better airflows and actually get about 10hp more as a result...they would set up and test the Gamijector flows so the engines would be perfectly balanced and the turbo fine tuned...just bolt em on and go...sorta. They would also come with liquidaire baffles which optimise airflow around the cylinders so you don't get a couple sitting 50F higher than the rest...it would be near bullet proof.
It aint cheap...but by the time you ordered IO550s locally and had the kit and injectors installed and fecked with to get it all running right it probably would be 6 of one and a dozen+ of the other.
If buying an older 58, nothing wrong with them, you might need to take advice on the engine mounts too.
Given the
incredibly depressed used market for any twin (actually any second hand aeroplane) at the moment I think you could be in a very strong position to get a good low time, relatively late model (late 70s/early 80s) 58 for a steal and turn it into an
absolute weapon.