Is my rear end strong enough for a 502 or am I gonna snap something?

Not folklore. A stock 10 bolt behind a big block with hooking tires is living on borrowed time, especally on a launch. The 12 bolt will hold up fine for a couple passes a month, so figuring out which one you have is the right move before you buy rubber.

Good news is you can eyeball it from under the car without pulling anything. Crawl back there and look at the shape of the rear cover. The 12 bolt cover is irregular, kind of squarish with rounded corners, and it has a raised bump on top where the pinion snubber mounts. The 10 bolt is basically a smooth oval with no bump. Bolt count alone will fool you because both have around ten cover bolts, so go by shape. Another giveaway is axle size. The 12 bolt tubes are fatter and the axle flanges are noticeably bigger if you want to compare measurements against a chart.

As for those stampings, look on the top of the passenger side axle tube, right where it meets the center section. There should be a two or three letter code stamped there. That code tells you the gear ratio and build date, and the lists are easy to find online. If there is no stamping, somebody may have swapped the center section at some point, which given the 502 swap history is entirely possable.

One more trick: the original paperwork or cowl tag won't tell you, but if the car still has its build sheet in it somewhere, that will list the axle code from the factory. Long shot on a car this age, but worth a look.

If it turns out to be a 10 bolt, don't panic. People run them behind mild big blocks, but you'd want to know exactly what gears and internals are in it before trusting it on a sticky tire launch. If it's a 12 bolt, send it.

One thing I'd add is you can nail down the gear ratio without any disassembly at all. Jack up one rear wheel, block the other, put the trans in neutral, mark the driveshaft, and turn the wheel two full revolutions while counting driveshaft rotations. Three and three quarters turns is roughly a 3.73, a hair over four is a 4.10, about three is a 3.08. I did this on my own car before I ever touched the cover and it matched the stamping exactly. Also, your El Camino could have been ordered with the stronger rear from the factory if it had the right options, so the previous owner's Chevelle story might not even be the whole picture.

Agree with the guy who said the 12 bolt will handle a couple passes a month, that matches what I saw. Mine lived behind a 454 in an old Nova for years of test and tune nights and never complained. Only thing I ever did was pull the cover the first winter I owned it, mostly out of curiosity, and it turned out the gears had been swapped at some point too. The oil that came out was dark and gritty, so fresh lube plus a new gasket made me feel a lot better about hammering it. If yours turns out to be the 12 bolt, that first cover pull tells you a lot about what the previous owner actually did back there.