The heads, if reworked, were probably not treated to a set of larger valves and the full-scale milling that would have been required to give markedly better performance. Those heads are not worth the effort. For a 400, there are a dozen better places to start.
The myriad of possible head choices are as varied as the opinions about which is best.
What you should be after (IMHO) is a reliable, streetable, torquey motor that runs all day long on pump gas and still reminds you that you are dirving a V-8 on street tires whenever you need to be reminded.
To get there from here, you will have to go after other heads. There's almost no two ways about it. What you are looking for is a set of heads that can be finished out to about 88-90cc combustion chambers, have large valves (2.11/1.77) and screw-in studs. There are many casting numbers that can be worked to achieve that, but only one set that actually came that way from the factory. (#64)
Here is a short list in the order that I like them. (We all have our biased opinions.)
#13 (Open up chambers) #62 (Open up chambers) #6x-4 (Mill) #96 (Mill) #4x-7H (Mill) #5C* (Mill)
There are others, but because of ambiguity in casting numbers (multiple years or multiple applications) I should't list them.
The #13 and #62 have superior ports, though the 6X are also good. To use these two heads the combustion chambers can be opened up by blending the corners and unshrouding the valves. This will increase the detonation resistance and promote better flow.
* #5C use a small exhaust valve. It's been shown to improve EGR which helps increase gas mileage with almost no compromise in performance. Edelbrock aluminum heads also use this same combination of small exhaust valve with large intake valve.
