340/360 Exhaust Manifold On A 318?

I have 340 exhaust manifolds on a 1970 318, I used 340 gaskets, no problems!!
I am doing this swap on a customer car now (1970 318), seems the gaskets(340) are so big at bottom on center port they won't be able to seal. Did you use a certain # ?
If I lay gasket(felpro) on head flat the port opening of the gasket is right on very bottom end of 318 port. Worried about no seal surface.