Dan gave good advice.
Besides that $500 bucks for a 396/402 even with heads and crank in my opinion is not a good deal for anyone but the person doing the selling. Not popular. Not wanted. Not much potential. This all means that in the performance world and in the market of buying and selling it is pratically worthless.
If you want an economical building block to use get any 454 block and a set of either 049 or 781 oval port heads. Then you have something with potential for not much cost. A 4.250 crank can really wake that one up. That block is strong enough to handle just about all you would expect of it.
Ed