Chaining Connections
This answer is based upon the wrong assumption that Technic chain links and conveyor-belt links (a.k.a. bulldozer chain links) connect to nothing except each other.
As remarked by Ivan Sanchez, this is contradicted by this set, which uses a connection between a conveyor-belt link and a standard arm/rod.

Assuming that these chain links only connect to each other, the only way to connect them to something else, is wrapping eight of them around a standard 16-cog gear. (You can also wrap six of them around an 8-cog gear, but I would not consider that a robust and intended connection.)

Finally, the gear can be connected to a length-3 axle with stud, which in turn can be connected to the bottom of the 2×4 brick.

For bulldozer chain links, this looks like this:





Now, it depends on how you interpret the challenge: You need nine other pieces to connect the chain link to the 2×4 brick. However, many of those pieces are again chain links and there are only two pieces between every chain link and the 2×4 brick.