blueprint for a 516 casting is 78cc. It is widely recognized, however, that most all heads were deeper than blueprint, and 83cc sounds about right. Conventional wisdom is that Ma did this on purpose figuring racers would work with their motors to bring them to blueprint, which would be higher compression than Mom's grocery-getter.

1.74 exhaust valves have most likely been installed at some point prior to you taking ownership, as I don't think any 516 castings came with them and it has been a fairly common thing to do since most BB heads have the 1.74 valves. With the 1.74's installed, they're pretty effective heads. Great for a 383 build, where you may not need the larger port flow #'s that a 906 casting offers, but do need the closed chamber to gather compression.