why would you need a forged rotating assembly if you don't plan on adding forced induction? The only reason you would need forged rods and pistons, etc is so you can rev higher. you would need strong rods as well as strong rod bolts for that. but don't forget your heads you have now would prolly need to be made a little larger for the exhaust ports. I don't know about the cam, but it seems kinda small and might run out of steam around 5500-6k rpms.
benefits are that you do it if the cylinder walls need to be bored due to wear. You can concievably get another 0.1L out of boring the engine, but it thins out the cylinder walls to where you only do it if you need to.
i would say that going to a 4.2L, you will see about 5-10% more torque and maybe 5% more hp. the main thing is the torque increase.
the cheapest way to do it, since you really don't need forged, would be to get a 4.2L engine from a 97 F150 and remove the rotating assy. and then install it on your car. If you can do the work yourself, it would cost about $1000 for the F150 engine.