I had the same problem, but with a motorcycle engine. I checked piston-to-valve clearance over and over and had plenty. Turns out the big $$ secret sauce valve springs were binding. Check everything, trust nothing!
I'm going to quote Woody here: "Trust, but verify."
And now I am going to quote myself: "There is a difference between being an 'engine assembler', and an 'engine builder'. You get to pick which one you are going to be . . . . Your wallet may have something to say about it though."
If you do not check every little stinkin' thing, well, that's on you.
I am always bemused by guys who brag about how fast they can put together an engine. (Notice I did not say assemble . . . .) In some cases, this can work out OK. But mostly, it doesn't work out very well . . . .
SlowengineassemblingFordboy