If the head's ID mark (the #2 or #4 up on the top corner of each head) has a dot next to it (like .4), it is an open chamber, which is not as desireable. They are "ok" for a numbers matching build, but from a performance standpoint, not so much. They are known for spark-knock, at least more so than the closed chamber 351c 4V heads were.
Now if I were building one, I'd have to greatly consider head choice as part of the build. If all I had was open chamber heads (2v or 4v) , I certainly would reconsider using a 351c based engine in favor of a 351w stroker, especially in a Fox body. Don't get me wrong, I love the 351c, but I also love other engines too including the 5.4 4v and 5.0 4v, however, it needs to be weighed: How much are you willing to spend, how much work at you willing to do in order to make it fit, against the cost of doing a 9.5 deck, or for that matter a 460-based engine, modular or anything else.
If they're 4v closed chambers, they could be worth building but again, have to look at cost vs budget (among many other things). The 4v closed chamber heads are good heads even by today's standards. Thing is, though, they are heavy, and can get expensive to re-work with new seats, guides, springs, valves, seals, rockers, etc--not counting any porting. While they ARE good heads, they aren't all that great on the exhaust side, which can be fixed (with more money). Cut the exhaust ports off and then put a set of port plates on, then blend & port normally. Helps them out dramatically. They can easily put as much into a set of 4v cleveland heads as a new set of aftermarket aluminum 9.5 deck heads cost brand new.