I had some Trans-Dapt valve covers once on Pro-Topline heads. Those covers required around 5/16"-3/8" gasket thickness to prevent leaks. This was due to the stamped gasket surfaces being too narrow and "off" by around 1/8". Great product, huh? Ended up going with a pair of cast aluminum covers, and was able to use a single gasket. BTW, I like the Fel-Pro gaskets with cork-rubber material bonded to both sides of a metal core.
I also had a fit issue with one of their alternator brackets where the holes were off by 1/4" or so. The Chinese QA standard must be +/- 1/4". I remember back when Trans-Dapt stuff was made in the USA and actually fit. Sad state of affairs these days.
On a final note, I have found the GM-licensed ProForm valve covers to fit like gloves. Had some chrome ones for over 10 years on a truck that sat outside, and never had any signs of rust trying to peek through. The metal is also heavier gauge than the cheapies. Made in Taiwan, as I recall. That's light years better than made in China, and applies to just any aftermarket parts -- metal panels, electrical switches, bumpers, etc.