have you got an air leak?
are all the connections back on the manifold ie brake servo pipe refitted, pressure regulator connection fitted, rubber cap on the pipe from the bottom of the throttle body/butterfly housing fitted.
i rebuilt a mini engine a few years ago and it would not start, thought that i'd got something really wrong as the vernier pullies i put on had the timing marks in the wrong place altogether.
just as i was resigning myself to the fact that i'd dropped a b*ll*ck and the engine would have to come out i spotted the hole in the inlet manifold where the servo would connect in an MG metro - put a bolt in the hole and it ran straight away.
Steve