I searched the site and found out that it's the pistons, not the heads that determine compression ratio. It's a 'red' 394 Hi comp, originally had the 10.25:1.

So my father-in-law had this engine rebuilt ~ 10k miles ago, but not sure what pistons they installed. Unfortunately he's not around anymore... He was a very practical Engineer, so I wouldn't be surprised if he asked for low-comp pistons on a rebuild in his later years He kept pretty meticulous records, but the bill of sale just says 8x pistons.

Is there any easy-ish way to tell what I've got?

 

Does it ping on anything but the highest grade of gas, if it does it's probably 10.25 to 1 compression. Years ago I had a 63 and It needed the best gas available and if I slipped a partial tank of mid range fuel in it it would ping going up a speed bump...Tedd

 

The car is stationary for now. I'm giving it 91 octane gas and it seems ok.

Wondering if there's a way I can figure out the compression ratio by measuring cylinder compression? Or feeling around to see if the pistons are dished...