Are you using stock bolts? If you're using aftermarket ones, they may be too long.

That being said, why don't the MP covers fit?

I went thru the same problem of leaking valve covers with my wagon. My wagon was originally a 400, but now has a (stock) 440 in it with stock HP manifolds. The extreme heat kept warping the stock covers and baking the gasket out. I bought a pair of used M/T covers from Zippy, installed the factory breather and PCV (with baffles) and haven't had a problem since. Which is why I'm wondering why M/T covers fit for me, but the MP covers don't fit for you?