assuming it hasn't been jacked with and is all original and due to the lack of finding the casting numbers i'd say it is a '70 or MAYBE a '71 302 but probably a '70.
i've come to this conclusion due to a number of things. mostly the valve covers and accessory brackets. Ford quit using that style of valve covers after the 70 model year also the brackets are 70 or newer the 69's used different style brackets. another thing i noticed was the wiring harness for the engine, the coil, coolant temp switch and oil pressure sender is the correct style for a 70 model, also it's a little hard to tell for sure but it looks like the oil pressure sending unit is a guage type and not an idiot light type so that leads me to beleive it is from a stang or a cougar as most other passenger cars in 70 had idiot lights, not all of them but a big chunk for sure.
and since you said it was supposedly in a 70 stang i'd have to say that is the original engine. of course without the casting numbers there is no way to be absolutely certain, that's just my best educated guess.
if you want to know for sure the block casting number will be on the block above the starter, you'll have to basically stand on your head to read them with the engine that close to the ground but that's the only way to know for sure.
if you want my advice though, i'd stay away from this one unless you have the money to have the entire block machined, it looks like it has been sitting outside for quite a while and is probably siezed up.