Why did the other one blow? Operator error/lack of maintaince? You need to know that to make a choice on the un blown one. IOW, if it was lack of maintaince, then the twin probably had the same. How many hours on it?
At the very least do a compression test on it, or better still, a leak down test. Pull the oil filter and cut it apart and look for junk in it.
The 502 Mag is a great engine with the best hi po Chevy parts in it. It takes a bunch of abuse to break a stock one, which is why I asked why the other one broke.