either he knows he is selling a heaver valve then stock and does not tell anyone or worse yet does not even know him self that the valve is a lot heaver then stock. ask him and find out. the machine shop should tell you if you need new guides and if your valve stems are out of spec. might just need new valve seals. since the intake valves which is the only valves that has valve seals and they are leaking. seems like if the valve guides are worn the exhaust valve would show some kind leaking too. like f 85 said my point was your valve springs might be weak and should be replaced. to put all back together and then have a miss from a weak and old valve spring. the heaver valve will put more pressure on your old valve springs to not have enough spring pressure.