Cylinder heads is where the HP is at. You can gain some HP by doing a valve job but 25 hp is unrealistic IMO. If you’re wanting to do some work yourself you can streamline the valve boss on the intake & exhaust runners. You can grind some off the push rod pinch point but not a lot, contour it some. Gasket matching the intake to the head usually doesn’t help much unless it’s really off but I do it any how. On the intake runners you want to leave a ruff surface like what 60 gr sandpaper roll would leave. Don’t do the exhaust like that only the pushrod boss shaping. A spring change might help to. LS6 springs with the correct retainers, there beehive springs, you can get GM LS6 springs for around 60.00.