Who can guess what happened to my 40?

[Gumby]

I would:

Pull the head apart and have it cleaned and checked for flatness or do those things yourself. If it needs it, take a couple of thousands off. Don't grind the valves unless a seat is burnt.

Clean the valves.
Replace the valve stem seals. The head kit will come with them and you have to take them off to clean the valves anyway.
Check the intake and make sure it's flat. You had some vacuum leaks. The intake, especially trying to seal with that stupid header, is a big source for those.
Put the fxxxer back together and run it until it pukes again.

 

[x factor]

I agree with Todd. I would also get some lapping compound and stick to check and clean the valves and seats and replace the valve springs as well. I am of the opinion that valves springs are overlooked far to often probably be cause of the stock car racing.