Okay so you can understand this better, 601 and 416 heads were both factory 305 heads the 416's are the better performing more coveted pair. I just looked up on Stan Weiss cylinder head flow chart and a pair of 416's with a mild port job only put out 180cfm @ .500" if the person who did the work did a good job then you may see 200cfm @ .500". This flow is what is hindering you do much. My estimate of 330-340hp is likely a push but you may be able to push this out. Remember these heads from the factory were used on an engine that was putting out less than 200hp. Is this seriously the head you want to try and make "good power" out of.
And yes, a 350 with a mildly ported set of .601 heads, a .230 degree cam... Would only make around 340-360hp. A motor is only as good as its heads. A motor could have a poor combination but a good set of heads and still make pretty decent power. But a motor with a well planned combination of parts with a mediocre set of heads will only make as much power as those heads will allow it to make. Never skimp out on you heads, on a decent motor it can literally be the difference of 100hp.