I think the most cost effective would be a hybrid (3x00 top end on your engine) or an entire 3100/3400 swap. If you did a 3400 swap, you not only would have a very nice base in which to start modifying (185hp @ 5200, 210 tqe @ 4000 ), but you would learn a lot in the process. For the 3400, you can look in the wreckers for one from a Venture mini van ( very common), and get the "chip" reprogrammed, and the engine mount from JBody performance, and look at the "sticky" at the top of the performance forum for a more complete list of parts that are required.