Lets say I lent someone a hundred thousand worth of dollars, and that was in 2000. On the contract that we signed, I stated that I can take anything from his belongings to be mine, if I deem it as of equal value (and if the government approves it IS of equal value). His house was proven as of equal value to the remaining amount of the loan, can I take the house?
Depends on the jurisdiction, but in most of the United States, almost certainly not.
You'd need to attach his house as collateral, which you can usually do with a signed writing that indicates the exchange of your loan for an interest in his house. But the writing needs to have a certain level of specificity; "all debtor's real property" would probably be sufficient, but "all of X's belongings" is probably not going to be enforceable.
Even after that, there would still be the question of perfecting your interest in the house and determining whether you or some other creditor has higher priority. If, for instance, X took out a bank loan to buy the house and defaulted on that loan, as well, I would expect the bank to end up with house, as well.