Another possibility is that one of the pushrods has slipped off its lifter. Mine did, after a crash.

Pull the valve covers (watch out for the gaskets (as Donkey Doctor suggests) and see if the lifters are seated OK.

If you decide to take the engine totally apart, you will need a timing tool. I can make one up for you for a couple of bucks and a SASE. I had to make one up for mine, and have some stock left over.

Hope this helps,

Bob