Why replace the valve springs?
You'll probably have to remove the head from the engine to swap them, as I've not seen a way to compress the springs with the head in place. On that same note, it's easier to remove the engine and yank the head than it is to try and leave it in place and wrestle it around the frame (if it will even come out that way).
So in essence, you're going to have to remove the engine to replace valve springs that are probably just fine... I'm still running the stockers with my Yosh cams, and know of lots of folks still successfully using stock ones with bigger cams...
Just my opinion, though...
