I don't know if the clearances are the same for the 400 and 450, but those numbers are very similar to what my manual said, so I'd say it sounds right. Double check in a manual first though. It's a really easy job, just take your time. I have a feeler gauge specifically for valves, so it has a bent angle at the end, and a feeler gauge that I use for spark plugs, so it is flat with no bend. I don't know if it is because the valve gauge is for cars, but it didn't have a small enough size to work on my 450 engine, so I had to use the straight one. Just a heads up, check that the gauge has the right size if you buy or borrow one.
For my top end rebuild, I'm not going to lie, I had a shop do it. I bought a full gasket set and just handed it to them with my engine, you can look at my thread where I talk about what the shop actually did when I got it back. Like you said, there are no instructions with the gasket kits (which makes no sense), but you can just match up old vs. new to see where the new gaskets should go. Check the valve guides too if you tear into it, those might be sloppy which could also cause the noise. I can never remember the name of the part, but the screw on the end of the rocker arm that contacts the end of the valve, the part that you adjust when you adjust the valves, check those too. They are cheap and could need replacement.
If you do tear into it, check clearances on everything. If anything is out of spec, that would be the time to replace it or fix it.
For my top end rebuild, I'm not going to lie, I had a shop do it. I bought a full gasket set and just handed it to them with my engine, you can look at my thread where I talk about what the shop actually did when I got it back. Like you said, there are no instructions with the gasket kits (which makes no sense), but you can just match up old vs. new to see where the new gaskets should go. Check the valve guides too if you tear into it, those might be sloppy which could also cause the noise. I can never remember the name of the part, but the screw on the end of the rocker arm that contacts the end of the valve, the part that you adjust when you adjust the valves, check those too. They are cheap and could need replacement.
If you do tear into it, check clearances on everything. If anything is out of spec, that would be the time to replace it or fix it.
but on the bright side, yours will be in tip top shape. If you don't mind throwing a video on here when you are done or in your page so I can hear yours compared to mine once the valves are adjusted.
