That "287" number alone makes you think it's a 287? GM used 3 digit codes all over engine blocks, I wouldn't expect that to be a 287...by that time, they would have been using liters as an indicator of size anyway. That 287 is going to be a mold ID or something equally mundane.
I have a feeling thats nothing more than some small bore SBC (262, 265, 305, etc) that just doesn't use a common (or one that anyone cares about) casting number and thats why no record online.
As someone else asked for, whats the suffix code on that small pad? If it's blank, it could also be a factory replacement.
If it is a lemans motor I'll bet money that the Lemans didn't have anything but standard GM engine fare that year. GM didn't waste time certifying seperate engines for emissions if they didn't have to. There were no "oh we have this lying around, lets drop it in" at that point in history.
The less options they had in the drivetrain, the less EPA certification hassles, and in the cars, GM certainly didn't offer any more engine options than they had to. Way too much speculation, at this point origin is heresay and not going to be proven unless the suffix code checks out.