your gunna need a tune after. yes you can use the stock valves. the reason you need to upgrade springs is because the lift on the new cams that you will be putting in will be much greater than the stock ones. the stock springs will not be able to keep up and then you float a valve. that sucks. so they make tigher springs that can handle the heavy lift.

you can get a compression tester without the guage on it and force a 140lbs of air in th cyl and change out springs without pulling the head if you have a valve spring comressor that work from the top like i did. the air will force the valves up and stay up in order to swap springs. this is the tool i used on my h22 in order not to pull the head after i built it and then decied to put valve seals in.

like i said i was able to force air in cyl using the compression tester and a air compressor and this tool from up top without taking the head off. oh and always made sure the valves were pretty much closed when i did this so youll have to turn the bottom half to get it where you want it.