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Well, it certainly looks like the accessory drive is from the 80s-90s..... One thing that can get you in a ballpark is to see if the driver side has the clutch arm boss tapped. If it's not there, it's definitely newer. Another indicator can be where the oil dipstick is located. If it's in the timing cover, it's very likely older, but if it's on the side of the block it's newer. It's hard to say where the aftermarket dipstick goes from the picture. One last thing that I can think of that doesn't require a lot of digging, check the firing order. Older 302s have a 1-5-4-2 firing order whereas later model HO engines use the 351W firing order that starts out as 1-3-7-2.
Beyond that, I think it'll take a little more digging. The casting numbers are above the starter, so pulling the starter and recording those numbers will tell you when the block was produced. You can also pull the valve covers and record the casting numbers there to find when they were produced.
I'm sure I'm forgetting or not thinking of other things that others may help with, but I think this stuff should be able to get you some info.