Someone already put big valves in this, I would point that out in your ad jefe de jefes, that is a good selling point, half the work is already done.
The W53 head usually has small valves and small intake runners, this one only has the small intake runners, makes it worth what you are asking, if I already didn't have a couple extra heads just sitting, I would likely buy this head, but I already have a completely rebuilt/modified W53 head, and a 219 head just waiting to be rebuilt.
BTW, I also have an open chamber W53 head, figure that one out.
“The difference between genius and stupidly is that genius has its limits” Albert Einstein