The links used to put Moussaoui in prison are as concrete as those showing links between Al Quaida terrorists and the OKC bombing, no more no less. If you believe one, you should believe the other.
And, like other famous incidences of terrorism that are covered up, there is a phsyical impossibility in the cover-up: No one man could mix that much ANFO in one night. If you have ever mixed a wheelbarrow load - a fraction of one 55 gallon drum - of cement with a shovel, you know why the suggestion that McVeigh mixed the whole bomb himself is impossible by an order of magnitude at least.
But that isn't a problem for Super Terrorist and his Invisible Helpers.