My 2 cents. Depending on how the car runs now would determine what changes you really need to make. Are you blowing smoke out the pipe when you start up? Is the engine misfiring? Has the engine felt like it lost power, especially on up gentle grades? Is there an engine knock? Does the cylinder head(s) emit a knock/tick? How old is your car? Has it been raced a lot? How good is your transmission, is it worn and restricting power transfer to the rear end? Does your engine lope at idle? If you want to rebuild, start with the heads. If you do rebuild, build up. If you can get more from an engine swap, do so unless you want to keep your stock engine. If you swap, do swap up to the 5.4 ltr. It's a beast.